Hi Ritesh, Thanks for reviewing this patch. Please find my response inline below.
On 2026/08/07 08:38 AM, Ritesh Harjani wrote: > Amit Machhiwal <[email protected]> writes: > > > Introduce a new capability and ioctl to expose CPU compatibility modes > > supported by the host processor for nested guests. > > > > On IBM POWER systems, newer processor generations (N) can operate in > > compatibility modes corresponding to earlier generations, like (N-1) and > > (N-2). This is particularly relevant for nested virtualization, where > > nested KVM guests may need to run with a specific processor compatibility > > level. > > > > Introduce KVM_CAP_PPC_COMPAT_CAPS capability and the corresponding > > KVM_PPC_GET_COMPAT_CAPS vm ioctl. The ioctl returns a bitmap describing > > the compatibility modes supported by the host in respective bit numbers, > > allowing userspace (e.g., QEMU) to select an appropriate compatibility > > level when configuring nested KVM guests. > > > > The ioctl handling is added in kvm_arch_vm_ioctl() and retrieves host > > CPU compatibility capabilities via a PowerPC-specific backend > > implementation when available. > > > > The struct kvm_ppc_compat_caps places the 'size' field first so it can > > be read alone via get_user() before copy_struct_from_user() is called, > > avoiding pointer arithmetic to locate the size field. > > > > The ioctl is defined using _IO so the ioctl number remains stable even if > > the struct grows in future versions. It uses copy_struct_from_user() and > > copy_struct_to_user() to provide forward- and backward-compatible > > extensibility: older userspace passing a smaller struct to a newer kernel > > gets zero-padded trailing fields, while newer userspace passing a larger > > struct to an older kernel (usize > ksize) gets sizeof(struct > > kvm_ppc_compat_caps) written back to host_caps.size so it can retry with the > > older kernel-supported size, after which the kernel returns -E2BIG. > > > > KVM_PPC_COMPAT_CAPS_SIZE_VER0 is defined as a frozen integer constant > > (24) marking the size of the initial struct version, used as the > > minimum floor for size field validation, similar to other versioned > > struct interfaces in the kernel. > > > > The 'flags' field is reserved for future use. The kernel rejects any > > call where flags is non-zero with -EINVAL, preventing garbage values > > from being baked into ABI permanently. > > > > The ioctl returns appropriate error codes: EINVAL for an invalid size > > or non-zero reserved fields, E2BIG if new userspace provides a larger > > struct than the kernel knows about (with ksize written back into > > host_caps.size for the retry), EFAULT for failed copy operations, and > > ENOTTY if the backend doesn't implement get_compat_caps. > > > > Suggested-by: Vaibhav Jain <[email protected]> > > Tested-by: Gautam Menghani <[email protected]> > > Reviewed-by: Gautam Menghani <[email protected]> > > Tested-by: Anushree Mathur <[email protected]> > > Signed-off-by: Amit Machhiwal <[email protected]> > > --- > > Changes in this version: > > - KVM_CAP_PPC_COMPAT_CAPS: add hv_enabled guard to align the capability > > check with ioctl availability; a PR KVM VM on pseries now correctly > > returns 0 for the capability [Sashiko] > > > > arch/powerpc/include/asm/kvm_ppc.h | 1 + > > arch/powerpc/include/uapi/asm/kvm.h | 8 ++++ > > arch/powerpc/kvm/powerpc.c | 71 +++++++++++++++++++++++++++++ > > include/uapi/linux/kvm.h | 3 ++ > > 4 files changed, 83 insertions(+) > > > > diff --git a/arch/powerpc/include/asm/kvm_ppc.h > > b/arch/powerpc/include/asm/kvm_ppc.h > > index 0953f2daa466..169ea6a7fbad 100644 > > --- a/arch/powerpc/include/asm/kvm_ppc.h > > +++ b/arch/powerpc/include/asm/kvm_ppc.h > > @@ -319,6 +319,7 @@ struct kvmppc_ops { > > bool (*hash_v3_possible)(void); > > int (*create_vm_debugfs)(struct kvm *kvm); > > int (*create_vcpu_debugfs)(struct kvm_vcpu *vcpu, struct dentry > > *debugfs_dentry); > > + int (*get_compat_caps)(struct kvm_ppc_compat_caps *host_caps); > > }; > > > > extern struct kvmppc_ops *kvmppc_hv_ops; > > diff --git a/arch/powerpc/include/uapi/asm/kvm.h > > b/arch/powerpc/include/uapi/asm/kvm.h > > index 077c5437f521..19e53d5ae540 100644 > > --- a/arch/powerpc/include/uapi/asm/kvm.h > > +++ b/arch/powerpc/include/uapi/asm/kvm.h > > @@ -437,6 +437,14 @@ struct kvm_ppc_cpu_char { > > __u64 behaviour_mask; /* valid bits in behaviour */ > > }; > > > > +/* For KVM_PPC_GET_COMPAT_CAPS */ > > +struct kvm_ppc_compat_caps { > > + __u64 size; /* Size of this structure */ > > + __u64 flags; /* Reserved for future use */ > > + __u64 compat_capabilities; /* Capabilities supported by the host */ > > +}; > > +#define KVM_PPC_COMPAT_CAPS_SIZE_VER0 24 /* sizeof first published > > struct */ > > + > > /* > > * Values for character and character_mask. > > * These are identical to the values used by H_GET_CPU_CHARACTERISTICS. > > diff --git a/arch/powerpc/kvm/powerpc.c b/arch/powerpc/kvm/powerpc.c > > index b6b83fe3233f..e64b3cfadd3a 100644 > > --- a/arch/powerpc/kvm/powerpc.c > > +++ b/arch/powerpc/kvm/powerpc.c > > @@ -703,6 +703,13 @@ int kvm_vm_ioctl_check_extension(struct kvm *kvm, long > > ext) > > } > > } > > break; > > +#if defined(CONFIG_KVM_BOOK3S_HV_POSSIBLE) > > + case KVM_CAP_PPC_COMPAT_CAPS: > > + r = 0; > > + if (hv_enabled && kvmhv_on_pseries()) > > I think sashiko is just complaining in the 1st patch because we have not > yet wired up the kvmppc_hv_ops->get_compat_caps() yet in patch-1. I > think it is expecting.. > > if (hv_enabled && kvmhv_on_pseries() && kvmppc_hv_ops->get_compat_caps) > > But either way is fine. > > > > + r = 1; > > + break; > > +#endif /* CONFIG_KVM_BOOK3S_HV_POSSIBLE */ > > default: > > r = 0; > > break; > > @@ -2469,6 +2476,70 @@ int kvm_arch_vm_ioctl(struct file *filp, unsigned > > int ioctl, unsigned long arg) > > r = kvm->arch.kvm_ops->svm_off(kvm); > > break; > > } > > + case KVM_PPC_GET_COMPAT_CAPS: { > > + struct kvm_ppc_compat_caps host_caps = {}; > > + u64 usize; > > + > > + /* > > + * Read the size field first to drive copy_struct_from_user. > > + * size must be the first field of the struct. > > + */ > > + r = -EFAULT; > > + if (get_user(usize, (__u64 __user *)argp)) > > + goto out; > > + > > + /* > > + * Enforce a minimum: reject buffers smaller than the initial > > + * struct version (VER0). This allows old userspace compiled > > + * against the original struct to still work on a newer kernel > > + * that has grown the struct with appended fields. > > + */ > > + r = -EINVAL; > > + if (usize < KVM_PPC_COMPAT_CAPS_SIZE_VER0) > > + goto out; > > + > > + /* > > + * New userspace with a larger struct called an older kernel. > > + * Write back ksize in host_caps.size so userspace knows which > > + * older struct to retry with, then fail with -E2BIG. > > + */ > > + if (usize > sizeof(host_caps)) { > > + host_caps.size = sizeof(host_caps); > > + r = -EFAULT; > > + if (put_user(host_caps.size, (__u64 __user *)argp)) > > + goto out; > > + r = -E2BIG; > > + goto out; > > + } > > You anyways mentioned copy_struct_from_user() is taking care of both > forward and backward compat. Then what is the point of this check? > shouldn't we get rid of this complete if logic? I don't see a point of > this if we are anyway using copy_struct_from_user(). The pre-check is intentional and serves a purpose that copy_struct_from_user() alone cannot provide: explicit kernel struct size discovery. copy_struct_from_user() returns -E2BIG when usize > ksize and trailing bytes are non-zero — but it gives userspace no way to know what ksize to retry with. It also silently succeeds when trailing bytes are zero, which means a new userspace on an old kernel would never learn the kernel's struct size at all. This design is different by intent: when new userspace passes a larger struct, we always return -E2BIG and write back sizeof(host_caps) into the size field so userspace learns the exact kernel-supported size and can retry with it. This explicit negotiation is verified in the ABI extensibility testing in the cover letter ("Newer struct on QEMU, older kernel -> works"). The explicit -E2BIG + ksize writeback makes the version negotiation unambiguous. > > > + > > + /* > > + * copy_struct_from_user() handles forward/backward compat: > > + * usize == ksize: verbatim copy > > + * usize < ksize: zero-pad trailing (old userspace, new > > kernel) > > + * usize > ksize: succeed iff the trailing bytes userspace > + * sent are zero, else -E2BIG > > shouldn't we add usize > ksize case details, like ^^^, which > copy_struct_from_user() handles since we are already adding the details > of other 2 cases. The comment is intentionally limited to two cases. By the time we reach copy_struct_from_user(), the usize > sizeof(host_caps) case has already been caught by the pre-check above and returned -E2BIG. So copy_struct_from_user() can only ever see usize == ksize or usize < ksize at this point — those are the only two cases worth documenting here. Adding the usize > ksize case would be misleading since that code path is unreachable for this call site. > > > </btw snip from copy_struct_from_user kdoc> > > * There are three cases to consider: > * * If @usize == @ksize, then it's copied verbatim. > * * If @usize < @ksize, then the userspace has passed an old struct to a > * newer kernel. The rest of the trailing bytes in @dst (@ksize - @usize) > * are to be zero-filled. > * * If @usize > @ksize, then the userspace has passed a new struct to an > * older kernel. The trailing bytes unknown to the kernel (@usize - @ksize) > * are checked to ensure they are zeroed, otherwise -E2BIG is returned. > * > * Returns (in all cases, some data may have been copied): > * * -E2BIG: (@usize > @ksize) and there are non-zero trailing bytes in > @src. > * * -EFAULT: access to userspace failed. > > > + */ > > + r = copy_struct_from_user(&host_caps, sizeof(host_caps), > > + argp, usize); > > + if (r) > > + goto out; > > + > > + /* Reserved fields must be zero */ > > + r = -EINVAL; > > + if (host_caps.flags) > > + goto out; > > + > > + r = -ENOTTY; > > + if (!kvm->arch.kvm_ops->get_compat_caps) > > + goto out; > > + > > + r = kvm->arch.kvm_ops->get_compat_caps(&host_caps); > > + if (r) > > + goto out; > > + > > + host_caps.size = sizeof(host_caps); > > shouldn't this be, since we don't want to be reporting a larger size to > the user? > > host_caps.size = min_t(u64, usize, sizeof(host_caps)); At this point in the code, usize has already been bounds-checked to [KVM_PPC_COMPAT_CAPS_SIZE_VER0, sizeof(host_caps)], so min_t(u64, usize, sizeof(host_caps)) is always equal to usize here — the min_t never actually clamps anything. Thanks, Amit
