Re: [PATCH v2] KVM: x86: Allow guests to see MSR_IA32_TSX_CTRL even if tsx=off

[Sean Christopherson]

On Mon, Feb 01, 2021, Paolo Bonzini wrote:
> On 29/01/21 17:58, Sean Christopherson wrote:
> > On Fri, Jan 29, 2021, Paolo Bonzini wrote:
> > >   	 */
> > >   	if (!boot_cpu_has(X86_FEATURE_RTM))
> > > -		data &= ~(ARCH_CAP_TAA_NO | ARCH_CAP_TSX_CTRL_MSR);
> > > +		data &= ~ARCH_CAP_TAA_NO;
> > 
> > Hmm, simply clearing TSX_CTRL will only preserve the host value.  Since
> > ARCH_CAPABILITIES is unconditionally emulated by KVM, wouldn't it make sense to
> > unconditionally expose TSX_CTRL as well, as opposed to exposing it only if it's
> > supported in the host?  I.e. allow migrating a TSX-disabled guest to a host
> > without TSX.  Or am I misunderstanding how TSX_CTRL is checked/used?
> 
> I'm a bit wary of having a combination (MDS_NO=0, TSX_CTRL=1) that does not
> exist on bare metal.  There are other cases where such combinations can
> happen, especially with the Spectre and SSBD mitigations (for example due to
> AMD CPUID bits for Intel processors), but at least those are just
> redundancies in the CPUID bits and it's more likely that the guest does
> something sensible with them.

Gotcha.  The vulnerability combos and all the double and triple negatives make
my head spin.

Thanks!