Re: Re: Re: Re: [PATCH] x86/bugs: KVM: Add support for SRSO_MSR_FIX

[Yosry Ahmed <yosry.ahmed@linux.dev>]

On Mon, Feb 17, 2025 at 05:07:28PM +0100, Borislav Petkov wrote:
> On Sun, Feb 16, 2025 at 09:59:59PM -0800, Yosry Ahmed wrote:
> > If 1-2% is the cost for keeping the MSR enabled at all times, I wonder
> > if we should just do that for simplicitly, and have it its own
> > mitigation option (chosen by the cmdline).
> 
> Yes, that's what David and I think we should do initially.
> 
> Then we can chase some more involved scheme like setting the bit before the
> first VM runs and then clearing it when the last one exits. I *think* I've
> seen something like that in KVM but I need to stare in detail.
> 
> > - Upon return to userspace (similar to your previous proposal). In this
> >   case we run userspace with the MSR cleared, and only perform an IBPB
> >   in the exit to userspace pace.
> 
> You want to IBPB before clearing the MSR as otherwise host kernel will be
> running with the mistrained gunk from the guest.

I meant IBPB + MSR clear before going to userspace, or IBPB + MSR clear
before a context switch.

> 
> > Any thoughts? 
> 
> Yes, let's keep it simple and do anything more involved *only* if it is really
> necessary.
> 
> -- 
> Regards/Gruss,
>     Boris.
> 
> https://people.kernel.org/tglx/notes-about-netiquette