Re: [PATCH] KVM: selftests: Make guest_code_xsave more friendly

[Sean Christopherson <seanjc@google.com>]

On Wed, Jun 03, 2026, Ackerley Tng via B4 Relay wrote:
> From: Ackerley Tng <ackerleytng@google.com>
> 
> The original implementation of guest_code_xsave makes a jmp to
> guest_sev_es_code in inline assembly. When code that uses guest_sev_es_code
> is removed, guest_sev_es_code will be optimized out, leading to a linking
> error since guest_code_xsave still tries to jmp to guest_sev_es_code.

So, don't do that?

> Rewrite guest_code_xsave() to instead make a call, in C, to
> guest_sev_es_code(), so that usage of guest_sev_es_code() is made known to
> the compiler.
> 
> This rewriting also gives a name to the xsave inline assembly, improving
> readability.
> 
> Signed-off-by: Ackerley Tng <ackerleytng@google.com>
> ---
>  tools/testing/selftests/kvm/x86/sev_smoke_test.c | 24 +++++++++++++++++-------
>  1 file changed, 17 insertions(+), 7 deletions(-)
> 
> diff --git a/tools/testing/selftests/kvm/x86/sev_smoke_test.c b/tools/testing/selftests/kvm/x86/sev_smoke_test.c
> index 1a49ee3915864..8b859adf4cf6f 100644
> --- a/tools/testing/selftests/kvm/x86/sev_smoke_test.c
> +++ b/tools/testing/selftests/kvm/x86/sev_smoke_test.c
> @@ -80,13 +80,23 @@ static void guest_sev_code(void)
>  	GUEST_DONE();
>  }
>  
> -/* Stash state passed via VMSA before any compiled code runs.  */

Uh, so as the comment says, the goal is to stash state before _any_ compiled
code runs.  Shoving the code into inline asm breaks that.  The compiler *probably*
won't shove anything before the first inline assembly, but there are absolutely
no guarantees.  E.g. you're subtly relying on a tail-call optimization to avoid
any stack operations.  If I force guest_sev_es_code() to be inlined, then the
prologue becomes:

0000000000402a10 <guest_code_xsave>:
  402a10:       48 83 ec 08                                     sub    $0x8,%rsp
  402a14:       b8 07 00 00 00                                  mov    $0x7,%eax
  402a19:       31 d2                                           xor    %edx,%edx
  402a1b:       0f ae 27                                        xsave  (%rdi)
  402a1e:       b9 31 01 01 c0                                  mov    $0xc0010131,%ecx

and we're hosed.

> -extern void guest_code_xsave(void);
> -asm("guest_code_xsave:\n"
> -    "mov $" __stringify(XFEATURE_MASK_X87_AVX) ", %eax\n"
> -    "xor %edx, %edx\n"
> -    "xsave (%rdi)\n"
> -    "jmp guest_sev_es_code");
> +static void xsave_all_registers(void *addr)
> +{
> +	__asm__ __volatile__(
> +		"mov $" __stringify(XFEATURE_MASK_X87_AVX) ", %eax\n"
> +		"xor %edx, %edx\n"

This doesn't even build.  When using input and/or output params, named registers
like eax and edx need an extra '%' to escape them, e.g.

	asm volatile("mov $" __stringify(XFEATURE_MASK_X87_AVX) ", %%eax\n\t"
		     "xor %%edx, %%edx\n\t"
		     "xsave (%0)"
		:
		: "r"(addr)
		: "eax", "edx", "memory"
	 );

> +		"xsave (%0)"
> +		:
> +		: "r"(addr)
> +		: "eax", "edx", "memory"
> +	 );
> +}
> +
> +static void guest_code_xsave(void *vmsa_gva)
> +{
> +	xsave_all_registers(vmsa_gva);
> +	guest_sev_es_code();
> +}
>  
>  static void compare_xsave(u8 *from_host, u8 *from_guest)
>  {
> 
> ---
> base-commit: 0d9b37717aaa4a73362520af5ba4db7febf09123
> change-id: 20260603-snp-selftest-cleanup-bf97734c6902
> 
> Best regards,
> --
> Ackerley Tng <ackerleytng@google.com>
> 
>