Re: [PATCH v3 06/10] KVM: selftests: Drop HORRIFIC_L2_UCALL_CLOBBER_HACK

[Sean Christopherson]

On Mon, Jun 29, 2026, Yosry Ahmed wrote:
> Now that nVMX test codes preserves GPRs across nested VM-Exits
> (specifically RBP, RDX, and RDI among others), drop the ucall-specific
> hack to avoid clobbering these registers.
> 
> Assisted-by: Gemini:gemini-3.1-pro

Did Gemini really help you write this patch?  It's ok if it did, I just want to
be precise with documenting how AI was used so that when I get a patch that was
indeed heavily helped by AI, I can actually key off the "AI was used" information
for context when reviewing.

> Signed-off-by: Yosry Ahmed <yosry@xxxxxxxxxx>
> ---
>  tools/testing/selftests/kvm/lib/x86/ucall.c | 32 ++-------------------
>  1 file changed, 2 insertions(+), 30 deletions(-)
> 
> diff --git a/tools/testing/selftests/kvm/lib/x86/ucall.c b/tools/testing/selftests/kvm/lib/x86/ucall.c
> index e7dd5791959ba..38050c60a0670 100644
> --- a/tools/testing/selftests/kvm/lib/x86/ucall.c
> +++ b/tools/testing/selftests/kvm/lib/x86/ucall.c
> @@ -10,36 +10,8 @@
>  
>  void ucall_arch_do_ucall(gva_t uc)
>  {
> -	/*
> -	 * FIXME: Revert this hack (the entire commit that added it) once nVMX
> -	 * preserves L2 GPRs across a nested VM-Exit.  If a ucall from L2, e.g.
> -	 * to do a GUEST_SYNC(), lands the vCPU in L1, any and all GPRs can be
> -	 * clobbered by L1.  Save and restore non-volatile GPRs (clobbering RBP
> -	 * in particular is problematic) along with RDX and RDI (which are
> -	 * inputs), and clobber volatile GPRs. *sigh*
> -	 */
> -#define HORRIFIC_L2_UCALL_CLOBBER_HACK	\
> -	"rcx", "rsi", "r8", "r9", "r10", "r11"
> -
> -	asm volatile("push %%rbp\n\t"
> -		     "push %%r15\n\t"
> -		     "push %%r14\n\t"
> -		     "push %%r13\n\t"
> -		     "push %%r12\n\t"
> -		     "push %%rbx\n\t"
> -		     "push %%rdx\n\t"
> -		     "push %%rdi\n\t"
> -		     "in %[port], %%al\n\t"
> -		     "pop %%rdi\n\t"
> -		     "pop %%rdx\n\t"
> -		     "pop %%rbx\n\t"
> -		     "pop %%r12\n\t"
> -		     "pop %%r13\n\t"
> -		     "pop %%r14\n\t"
> -		     "pop %%r15\n\t"
> -		     "pop %%rbp\n\t"
> -		: : [port] "d" (UCALL_PIO_PORT), "D" (uc) : "rax", "memory",
> -		     HORRIFIC_L2_UCALL_CLOBBER_HACK);
> +	asm volatile("in %[port], %%al"
> +		: : [port] "d" (UCALL_PIO_PORT), "D" (uc) : "rax", "memory");
>  }
>  
>  void *ucall_arch_get_ucall(struct kvm_vcpu *vcpu)
> -- 
> 2.55.0.rc0.799.gd6f94ed593-goog
>