Re: [PATCH] x86/retpoline: Don't clobber RFLAGS during srso_safe_ret()

[Sean Christopherson]

On Fri, Aug 11, 2023, Borislav Petkov wrote:
> On Fri, Aug 11, 2023 at 08:52:55AM -0700, Sean Christopherson wrote:
> > A major motivation for doing fast emulation is to leverage the CPU to
> > handle consumption and manipulation of arithmetic flags, i.e. RFLAGS is
> > both an input and output to the target of the call.  fastop() collects
> > the RFLAGS result by pushing RFLAGS onto the stack and popping them back
> > into a variable (held in RDI in this case)
> > 
> >   asm("push %[flags]; popf; " CALL_NOSPEC " ; pushf; pop %[flags]\n"
> 
> Right, and I've tested this countless times with gcc-built host and
> guest.
> 
> But Nathan's case where the host is built with gcc but the guest with
> clang, would trigger this. And as he confirms, that fixes it so I wonder
> what is the difference in code generation to make this rFLAGS corruption
> noticeable in that particular configuration.

Might be I/O APIC accesses?  Unless things have changed, the I/O APIC code uses
a struct overlay to access the I/O APIC, i.e. when doing emulated MMIO accesses.
If clang generates an ADD or whatever and consumes flags, e.g. instead of a
straight MOV, that would explain the problems.