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

[Sean Christopherson]

On Fri, Jun 05, 2026, Ackerley Tng wrote:
> Sean Christopherson <seanjc@xxxxxxxxxx> writes:
> > 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.
> >
> 
> And omg, I thought I was running the tests!!

FWIW, you probably were, I had to force it to be inline via __always_inline.

> >> +		"xsave (%0)"
> >> +		:
> >> +		: "r"(addr)
> >> +		: "eax", "edx", "memory"
> >> +	 );
> >> +}
> >> +
> >>
> >> [...snip...]
> >>
> 
> My bad, this patch is such a goof!
> 
> I still feel that it'd be nice to allow commenting out functions parts
> of selftests while developing other parts, but let's shelve this for
> now.

I mean, it's always "allowed", but that's firmly "Here be dragons!" territory.
There are any number of things that can go sideways when adding and/or removing
code for development and debug purposes, many of which are far more nefarious
and subtle than obscure compiler errors.