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From:   Paolo Bonzini <pbonzini@redhat.com>
Subject: Re: [PATCH 4/4] x86/tsx: Add cmdline tsx=fake to not clear CPUID bits
 RTM and HLE
Message-ID: <671be35f-220a-f583-aa31-3a2da7dae93a@redhat.com>
Date:   Tue, 6 Jul 2021 23:58:08 +0200
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On 06/07/21 23:33, Eduardo Habkost wrote:
> On Tue, Jul 6, 2021 at 5:05 PM Paolo Bonzini <pbonzini@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.

Paolo