Hi, > On Oct 7, 2023, at 02:54, Andrew Cooper <[email protected]> wrote: > > From: Michal Orzel <[email protected]> > > The x86 hypercall ABI really used to have 6-argument hypercalls. V4V, the > downstream predecessor to Argo did take 6th args. > > However, the 6th arg being %ebp in the 32bit ABI makes it unusable in > practice, because that's the frame pointer in builds with frame pointers > enabled. Therefore Argo was altered to being a 5-arg hypercall when it was > upstreamed. > > c/s 2f531c122e95 ("x86: limit number of hypercall parameters to 5") removed > the ability for hypercalls to take 6 arguments. > > Update the documentation to match reality. > > Signed-off-by: Michal Orzel <[email protected]> > Signed-off-by: Andrew Cooper <[email protected]>
Release-acked-by: Henry Wang <[email protected]> Kind regards, Henry > --- > CC: George Dunlap <[email protected]> > CC: Jan Beulich <[email protected]> > CC: Stefano Stabellini <[email protected]> > CC: Wei Liu <[email protected]> > CC: Julien Grall <[email protected]> > CC: Michal Orzel <[email protected]> > CC: Henry Wang <[email protected]> > > v2: > * Extend with the historical context of how 6-arg hypercalls have come and > gone. > --- > docs/guest-guide/x86/hypercall-abi.rst | 15 +++++++++++---- > xen/include/public/arch-x86/xen-x86_32.h | 2 +- > xen/include/public/arch-x86/xen-x86_64.h | 2 +- > 3 files changed, 13 insertions(+), 6 deletions(-) > > diff --git a/docs/guest-guide/x86/hypercall-abi.rst > b/docs/guest-guide/x86/hypercall-abi.rst > index 42a820386b68..c7a11a76712f 100644 > --- a/docs/guest-guide/x86/hypercall-abi.rst > +++ b/docs/guest-guide/x86/hypercall-abi.rst > @@ -4,7 +4,7 @@ Hypercall ABI > ============= > > Hypercalls are system calls to Xen. Two modes of guest operation are > -supported, and up to 6 individual parameters are supported. > +supported, and up to 5 individual parameters are supported. > > Hypercalls may only be issued by kernel-level software [#kern]_. > > @@ -18,17 +18,17 @@ The registers used for hypercalls depends on the > operating mode of the guest. > > * - ABI > - Hypercall Index > - - Parameters (1 - 6) > + - Parameters (1 - 5) [#params]_ > - Result > > * - 64bit > - RAX > - - RDI RSI RDX R10 R8 R9 > + - RDI RSI RDX R10 R8 > - RAX > > * - 32bit > - EAX > - - EBX ECX EDX ESI EDI EBP > + - EBX ECX EDX ESI EDI > - EAX > > 32 and 64bit PV guests have an ABI fixed by their guest type. The ABI for an > @@ -119,6 +119,13 @@ means. > .. [#kern] For HVM guests, ``HVMOP_guest_request_vm_event`` may be configured > to be usable from userspace, but this behaviour is not default. > > +.. [#params] Xen's ABI used to declare support for 6 hypercall arguments, > + using ``r9`` and ``ebp``. However, such an ABI clobbers the frame pointer > + in the 32bit code and does interact nicely with guest-side debugging. The > + predecessor to ``HYPERCALL_argo_op`` was a 6-argument hypercall, but the > + ABI was intentionally altered when Argo was upstreamed (Xen 4.13) to be > the > + 5-argument hypercall it now is. > + > .. [#mode] While it is possible to use compatibility mode segments in a 64bit > kernel, hypercalls issues from such a mode will be interpreted with the > 32bit ABI. Such a setup is not expected in production scenarios. > diff --git a/xen/include/public/arch-x86/xen-x86_32.h > b/xen/include/public/arch-x86/xen-x86_32.h > index 139438e83534..9e3bf06b121e 100644 > --- a/xen/include/public/arch-x86/xen-x86_32.h > +++ b/xen/include/public/arch-x86/xen-x86_32.h > @@ -12,7 +12,7 @@ > > /* > * Hypercall interface: > - * Input: %ebx, %ecx, %edx, %esi, %edi, %ebp (arguments 1-6) > + * Input: %ebx, %ecx, %edx, %esi, %edi (arguments 1-5) > * Output: %eax > * Access is via hypercall page (set up by guest loader or via a Xen MSR): > * call hypercall_page + hypercall-number * 32 > diff --git a/xen/include/public/arch-x86/xen-x86_64.h > b/xen/include/public/arch-x86/xen-x86_64.h > index 5d9035ed2230..43f6e3d22001 100644 > --- a/xen/include/public/arch-x86/xen-x86_64.h > +++ b/xen/include/public/arch-x86/xen-x86_64.h > @@ -12,7 +12,7 @@ > > /* > * Hypercall interface: > - * Input: %rdi, %rsi, %rdx, %r10, %r8, %r9 (arguments 1-6) > + * Input: %rdi, %rsi, %rdx, %r10, %r8 (arguments 1-5) > * Output: %rax > * Access is via hypercall page (set up by guest loader or via a Xen MSR): > * call hypercall_page + hypercall-number * 32 > -- > 2.30.2 >
