Re: [PATCH v3 12/13] KVM: selftests: Dedup assembly code for VMLAUNCH and VMRESUME

[Sean Christopherson <seanjc@google.com>]

On Thu, Aug 27, 2026, Yosry Ahmed wrote:
> On Thu, Aug 27, 2026 at 10:17 AM Sean Christopherson <seanjc@google.com> wrote:
> > eVMCS is a memory operand, i.e. it's MOV RSP, [evmcs->host_rip].  The existing
> > evmcs_{vmresume,vmlaunch}() code is rather stupid and loads the address into a
> > register, and then manually encodes MOV RSP, [<reg>].
> 
> Can't we keep it as a register operand just to avoid vmwrite_operand?
> Does it actually hurt in any way?

...

> > diff --git tools/testing/selftests/kvm/lib/x86/vmx.c tools/testing/selftests/kvm/lib/x86/vmx.c
> > index 1a8515de42b0..b5efd10950c7 100644
> > --- tools/testing/selftests/kvm/lib/x86/vmx.c
> > +++ tools/testing/selftests/kvm/lib/x86/vmx.c
> > @@ -179,7 +179,10 @@ void load_vmcs(struct vmx_pages *vmx)
> >         vmclear(vmx->shadow_vmcs_gpa);
> >  }
> >
> > -#define __BUILD_VMX_VM_ENTRY_HELPER(insn, prefix, vmwrite_insn, vmwrite_operand,       \
> > +const u64 HOST_RSP_ENCODING = HOST_RSP;
> > +const u64 HOST_RIP_ENCODING = HOST_RIP;
> > +
> > +#define __BUILD_VMX_VM_ENTRY_HELPER(insn, prefix, vmwrite_insn,        \
> >                                     __host_rsp, __host_rip)                             \
> >  static int __##prefix##_##insn(void)                                                   \
> >  {                                                                                      \
> > @@ -196,16 +199,17 @@ static int __##prefix##_##insn(void)                                                      \
> >                              VMX_SWITCH_GPRS_ASM                                        \
> >                              "pop %%rax;"                                               \
> >                              : [ret]"=&a"(ret)                                          \
> > -                            : [host_rsp]__stringify(vmwrite_operand)(__host_rsp),      \
> > -                              [host_rip]__stringify(vmwrite_operand)(__host_rip),      \
> > +                            : [host_rsp]"m"(__host_rsp),                               \
> > +                              [host_rip]"m"(__host_rip),                               \
> 
> I suppose we can use "r" here though?

No, because then the encoding for VMWRITE needs to be:

  vmwrite %%rsp, %[host_rsp]

but for MOV/eVMCS needs to be:

  mov %%rsp, (%[host_rsp])

Have fun feeding the '(' and ')' into the asm blob :-)