On 04.10.2013, at 13:45, Paul Mackerras wrote: > When an interrupt or exception happens in the guest that comes to the > host, the CPU goes to hypervisor real mode (MMU off) to handle the > exception but doesn't change the MMU context. After saving a few > registers, we then clear the "in guest" flag. If, for any reason, > we get an exception in the real-mode code, that then gets handled > by the normal kernel exception handlers, which turn the MMU on. This > is disastrous if the MMU is still set to the guest context, since we > end up executing instructions from random places in the guest kernel > with hypervisor privilege. > > In order to catch this situation, we define a new value for the "in guest" > flag, KVM_GUEST_MODE_HOST_HV, to indicate that we are in hypervisor real > mode with guest MMU context. If the "in guest" flag is set to this value, > we branch off to an emergency handler. For the moment, this just does > a branch to self to stop the CPU from doing anything further.
I don't understand how you get there. The only case I can imagine where you'd hit a normal Linux handler while in guest MMU context is a bug in the complex real mode handling code. So basically what you're doing is you're changing the "guest mode" bit to HOST_NV while you're executing these. The other change this patch does is it postpones the return to GUEST_MODE_NONE to after fast-path handling of interrupt exits. What if you simply don't introduce a new mode but instead only postpone the GUEST_MODE_NONE switch to later? Worst case that can happen is that your bug spins the CPU into handling that exit in a tight loop - not much different from your explicit spin, no? Alex > > While we're here, we define another new flag value to indicate that we > are in a HV guest, as distinct from a PR guest. This will be useful > when we have a kernel that can support both PR and HV guests concurrently. > > Signed-off-by: Paul Mackerras <[email protected]> > --- > arch/powerpc/include/asm/kvm_asm.h | 2 ++ > arch/powerpc/kvm/book3s_hv_rmhandlers.S | 31 +++++++++++++++++++++++++------ > 2 files changed, 27 insertions(+), 6 deletions(-) > > diff --git a/arch/powerpc/include/asm/kvm_asm.h > b/arch/powerpc/include/asm/kvm_asm.h > index e2d4d46..1bd92fd 100644 > --- a/arch/powerpc/include/asm/kvm_asm.h > +++ b/arch/powerpc/include/asm/kvm_asm.h > @@ -138,6 +138,8 @@ > #define KVM_GUEST_MODE_NONE 0 > #define KVM_GUEST_MODE_GUEST 1 > #define KVM_GUEST_MODE_SKIP 2 > +#define KVM_GUEST_MODE_GUEST_HV 3 > +#define KVM_GUEST_MODE_HOST_HV 4 > > #define KVM_INST_FETCH_FAILED -1 > > diff --git a/arch/powerpc/kvm/book3s_hv_rmhandlers.S > b/arch/powerpc/kvm/book3s_hv_rmhandlers.S > index 8e0f28f..f1f1bf3 100644 > --- a/arch/powerpc/kvm/book3s_hv_rmhandlers.S > +++ b/arch/powerpc/kvm/book3s_hv_rmhandlers.S > @@ -383,6 +383,9 @@ END_FTR_SECTION_IFSET(CPU_FTR_ARCH_206) > mtspr SPRN_DAR, r5 > mtspr SPRN_DSISR, r6 > > + li r6, KVM_GUEST_MODE_HOST_HV > + stb r6, HSTATE_IN_GUEST(r13) > + > BEGIN_FTR_SECTION > /* Restore AMR and UAMOR, set AMOR to all 1s */ > ld r5,VCPU_AMR(r4) > @@ -682,7 +685,7 @@ fast_guest_return: > mtspr SPRN_HSRR1,r11 > > /* Activate guest mode, so faults get handled by KVM */ > - li r9, KVM_GUEST_MODE_GUEST > + li r9, KVM_GUEST_MODE_GUEST_HV > stb r9, HSTATE_IN_GUEST(r13) > > /* Enter guest */ > @@ -742,6 +745,14 @@ kvmppc_interrupt: > */ > /* abuse host_r2 as third scratch area; we get r2 from PACATOC(r13) */ > std r9, HSTATE_HOST_R2(r13) > + > + lbz r9, HSTATE_IN_GUEST(r13) > + cmpwi r9, KVM_GUEST_MODE_HOST_HV > + beq kvmppc_bad_host_intr > + /* We're now back in the host but in guest MMU context */ > + li r9, KVM_GUEST_MODE_HOST_HV > + stb r9, HSTATE_IN_GUEST(r13) > + > ld r9, HSTATE_KVM_VCPU(r13) > > /* Save registers */ > @@ -793,10 +804,6 @@ END_FTR_SECTION_IFSET(CPU_FTR_HAS_PPR) > std r3, VCPU_GPR(R13)(r9) > std r4, VCPU_LR(r9) > > - /* Unset guest mode */ > - li r0, KVM_GUEST_MODE_NONE > - stb r0, HSTATE_IN_GUEST(r13) > - > stw r12,VCPU_TRAP(r9) > > /* Save HEIR (HV emulation assist reg) in last_inst > @@ -1186,6 +1193,10 @@ BEGIN_FTR_SECTION > mtspr SPRN_AMR,r6 > END_FTR_SECTION_IFSET(CPU_FTR_ARCH_206) > > + /* Unset guest mode */ > + li r0, KVM_GUEST_MODE_NONE > + stb r0, HSTATE_IN_GUEST(r13) > + > /* Switch DSCR back to host value */ > BEGIN_FTR_SECTION > mfspr r8, SPRN_DSCR > @@ -1388,7 +1399,7 @@ fast_interrupt_c_return: > stw r8, VCPU_LAST_INST(r9) > > /* Unset guest mode. */ > - li r0, KVM_GUEST_MODE_NONE > + li r0, KVM_GUEST_MODE_HOST_HV > stb r0, HSTATE_IN_GUEST(r13) > b guest_exit_cont > > @@ -1937,3 +1948,11 @@ END_FTR_SECTION_IFSET(CPU_FTR_ALTIVEC) > lwz r7,VCPU_VRSAVE(r4) > mtspr SPRN_VRSAVE,r7 > blr > + > +/* > + * We come here if we get any exception or interrupt while we are > + * executing host real mode code while in guest MMU context. > + * For now just spin, but we should do something better. > + */ > +kvmppc_bad_host_intr: > + b . > -- > 1.8.4.rc3 > -- To unsubscribe from this list: send the line "unsubscribe kvm-ppc" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html
