Re: [PATCH] KVM: SVM: Add register operand to vmsave call in sev_es_vcpu_load

[Sean Christopherson <seanjc@google.com>]

+Michael, as this will conflict with an in-progress series to use VMSAVE in the
common SVM run path.

https://lkml.kernel.org/r/20201214174127.1398114-1-michael.roth@amd.com

On Mon, Dec 21, 2020, Sean Christopherson wrote:
> On Fri, Dec 18, 2020, Nathan Chancellor wrote:
> > When using LLVM's integrated assembler (LLVM_IAS=1) while building
> > x86_64_defconfig + CONFIG_KVM=y + CONFIG_KVM_AMD=y, the following build
> > error occurs:
> > 
> >  $ make LLVM=1 LLVM_IAS=1 arch/x86/kvm/svm/sev.o
> >  arch/x86/kvm/svm/sev.c:2004:15: error: too few operands for instruction
> >          asm volatile(__ex("vmsave") : : "a" (__sme_page_pa(sd->save_area)) : "memory");
> >                       ^
> >  arch/x86/kvm/svm/sev.c:28:17: note: expanded from macro '__ex'
> >  #define __ex(x) __kvm_handle_fault_on_reboot(x)
> >                  ^
> >  ./arch/x86/include/asm/kvm_host.h:1646:10: note: expanded from macro '__kvm_handle_fault_on_reboot'
> >          "666: \n\t"                                                     \
> >                  ^
> >  <inline asm>:2:2: note: instantiated into assembly here
> >          vmsave
> >          ^
> >  1 error generated.
> > 
> > This happens because LLVM currently does not support calling vmsave
> > without the fixed register operand (%rax for 64-bit and %eax for
> > 32-bit). This will be fixed in LLVM 12 but the kernel currently supports
> > LLVM 10.0.1 and newer so this needs to be handled.
> > 
> > Add the proper register using the _ASM_AX macro, which matches the
> > vmsave call in vmenter.S.
> 
> There are also two instances in tools/testing/selftests/kvm/lib/x86_64/svm.c
> that likely need to be fixed.
>  
> > Fixes: 861377730aa9 ("KVM: SVM: Provide support for SEV-ES vCPU loading")
> > Link: https://reviews.llvm.org/D93524
> > Link: https://github.com/ClangBuiltLinux/linux/issues/1216
> > Signed-off-by: Nathan Chancellor <natechancellor@gmail.com>
> > ---
> >  arch/x86/kvm/svm/sev.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
> > index e57847ff8bd2..958370758ed0 100644
> > --- a/arch/x86/kvm/svm/sev.c
> > +++ b/arch/x86/kvm/svm/sev.c
> > @@ -2001,7 +2001,7 @@ void sev_es_vcpu_load(struct vcpu_svm *svm, int cpu)
> >  	 * of which one step is to perform a VMLOAD. Since hardware does not
> >  	 * perform a VMSAVE on VMRUN, the host savearea must be updated.
> >  	 */
> > -	asm volatile(__ex("vmsave") : : "a" (__sme_page_pa(sd->save_area)) : "memory");
> > +	asm volatile(__ex("vmsave %%"_ASM_AX) : : "a" (__sme_page_pa(sd->save_area)) : "memory");
> 
> I vote to add a helper in svm.h to encode VMSAVE, even if there is only the one
> user.  Between the rAX behavior (it _must_ be rAX) and taking the HPA of the
> VMCB, the semantics of VMSAVE are just odd enough to cause a bit of head
> scratching when reading the code for the first time.  E.g. something like:
> 
> void vmsave(struct page *vmcb)
> {
> 	/*
> 	 * VMSAVE takes the HPA of a VMCB in rAX (hardcoded by VMSAVE itself).
> 	 * The _ASM_AX operand is required to specify the address size, which
> 	 * means VMSAVE cannot consume a 64-bit address outside of 64-bit mode.
> 	 */
> 	hpa_t vmcb_pa = __sme_page_pa(vmcb);
> 
> 	BUG_ON(!IS_ENABLED(CONFIG_X86_64) && (vmcb_pa >> 32));
> 
> 	asm volatile(__ex("vmsave %%"_ASM_AX) : : "a" (vmcb_pa) : "memory");
> }
> 
> >  
> >  	/*
> >  	 * Certain MSRs are restored on VMEXIT, only save ones that aren't
> > -- 
> > 2.30.0.rc0
> >