Re: [PATCH 4/4] x86/tsx: Add cmdline tsx=fake to not clear CPUID bits RTM and HLE
On Wed, Jul 7, 2021 at 8:09 AM Eduardo Habkost <ehabkost(a)redhat.com> wrote:
CCing libvir-list, Jiri Denemark, Michal Privoznik, so they are aware
that the definition of "supported CPU features" will probably become a
bit more complex in the future.
Has there ever been a clear definition? Family, model, and stepping,
for instance: are these the only values supported? That would make
cross-platform migration impossible. What about the vendor string? Is
that the only value supported? That would make cross-vendor migration
impossible. For the maximum input value for basic CPUID information
(CPUID.0H:EAX), is that the only value supported, or is it the maximum
value supported? On the various individual feature bits, does a '1'
imply that '0' is also supported, or is '1' the only value supported?
What about the feature bits with reversed polarity (e.g.
CPUID.(EAX=07H,ECX=0):EBX.FDP_EXCPTN_ONLY[bit 6])?
This API has never made sense to me. I have no idea how to interpret
what it is telling me.
On Tue, Jul 6, 2021 at 5:58 PM Paolo Bonzini
<pbonzini(a)redhat.com> wrote:
>
> On 06/07/21 23:33, Eduardo Habkost wrote:
> > On Tue, Jul 6, 2021 at 5:05 PM Paolo Bonzini <pbonzini(a)redhat.com>
wrote:
> >> 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.
> >
> > Is the purpose of GET_SUPPORTED_CPUID to return what is supported by
> > KVM, or to return what "-cpu host" should enable by default? They
are
> > conflicting requirements in this case.
>
> In theory there is GET_EMULATED_CPUID for the former, so it should be
> the latter. In practice neither QEMU nor Libvirt use it; maybe now we
> have a good reason to add it, but note that userspace could also check
> host RTM_ALWAYS_ABORT.
>
> > Returning HLE=1,RTM=1 in GET_SUPPORTED_CPUID makes existing userspace
> > take bad decisions until it's updated.
> >
> > Returning HLE=0,RTM=0 in GET_SUPPORTED_CPUID prevents existing
> > userspace from resuming existing VMs (despite being technically
> > possible).
> >
> > The first option has an easy workaround that doesn't require a
> > software update (disabling HLE/RTM in the VM configuration). The
> > second option doesn't have a workaround. I'm inclined towards the
> > first option.
>
> The default has already been tsx=off for a while though, so checking
> either GET_EMULATED_CPUID or host RTM_ALWAYS_ABORT in userspace might
> also be feasible for those that are still on tsx=on.
This sounds like a perfect use case for GET_EMULATED_CPUID. My only
concern is breaking existing userspace.
But if this was already broken for a few kernel releases due to
tsx=off being the default, maybe GET_EMULATED_CPUID will be a
reasonable approach.
--
Eduardo