Re: [PATCH 4/4] x86/tsx: Add cmdline tsx=fake to not clear CPUID bits RTM and HLE

[Paolo Bonzini]

On 06/07/21 21:52, Eduardo Habkost wrote:
> On Wed, Jun 09, 2021 at 02:14:39PM -0700, Pawan Gupta wrote:
> > On CPUs that deprecated TSX, clearing the enumeration bits CPUID.RTM and
> > CPUID.HLE may not be desirable in some corner cases. Like a saved guest
> > would refuse to resume if it was saved before the microcode update
> > that deprecated TSX.
> Why is a global option necessary to allow those guests to be
> resumed?  Why can't KVM_GET_SUPPORTED_CPUID always return the HLE
> and RTM bits as supported when the host CPU has them?

It's a bit tricky, because HLE and RTM won't really behave well.  An old 
guest that sees RTM=1 might end up retrying and aborting transactions 
too much.  So I'm not sure that a QEMU "-cpu host" guest should have HLE 
and RTM enabled.

So it makes sense to handle it in userspace, with one of the two 
following possibilities:

- userspace sees TSX_FORCE_ABORT and if so it somehow "discourages" 
setting HLE/RTM, even though they are shown as supported

- userspace sees TSX_FORCE_ABORT and if so it knows HLE/RTM can be set, 
even though they are discouraged in general

In any case, KVM's "supported CPUID" is based on the host features but 
independent.  KVM can decide to show or hide the hardware HLE and RTM 
bits independent of the host tsx= setting; it may make sense to hide the 
bits via a module parameter, but in any case this patch is not needed.

Paolo