Re: [PATCH v2 00/12] x86/kvm/emulate: Avoid RET for FASTOPs

[Sean Christopherson <seanjc@google.com>]

On Mon, Nov 11, 2024, Peter Zijlstra wrote:
> Hi!
> 
> At long last, a respin of these patches.
> 
> The FASTOPs are special because they rely on RET to preserve CFLAGS, which is a
> problem with all the mitigation stuff. Also see things like: ba5ca5e5e6a1
> ("x86/retpoline: Don't clobber RFLAGS during srso_safe_ret()").
> 
> Rework FASTOPs to no longer use RET and side-step the problem of trying to make
> the various return thunks preserve CFLAGS for just this one case.
> 
> There are two separate instances, test_cc() and fastop(). The first is
> basically a SETCC wrapper, which seems like a very complicated (and somewhat
> expensive) way to read FLAGS. Instead use the code we already have to emulate
> JCC to fully emulate the instruction.
> 
> That then leaves fastop(), which when marked noinline is guaranteed to exist
> only once. As such, CALL+RET isn't needed, because we'll always be RETurning to
> the same location, as such replace with JMP+JMP.
> 
> My plan is to take the objtool patches through tip/objtool/core, the nospec
> patches through tip/x86/core and either stick the fastop patches in that latter
> tree if the KVM folks agree, or they can merge the aforementioned two branches
> and then stick the patches on top, whatever works for people.

Unless Paolo objects, I think it makes sense to take the fastop patches through
tip/x86/core.