Messages in this thread | | | Date | Fri, 11 Aug 2023 19:59:57 -0700 | Subject | Re: [PATCH] x86/retpoline: Don't clobber RFLAGS during srso_safe_ret() | From | Sean Christopherson <> |
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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.
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