On Wed, Jun 03, 2026, Ackerley Tng via B4 Relay wrote: > From: Ackerley Tng <[email protected]> > > 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 <[email protected]> > --- > 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 <[email protected]> > >
