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From:   Eduardo Habkost <ehabkost@redhat.com>
Date:   Tue, 6 Jul 2021 17:33:19 -0400
Message-ID: <CAOpTY_qdbbnauTkbjkz+cZmo8=Hz6qqLNY6i6uamqhcty=Q1sw@mail.gmail.com>
Subject: Re: [PATCH 4/4] x86/tsx: Add cmdline tsx=fake to not clear CPUID bits
 RTM and HLE
To:     Paolo Bonzini <pbonzini@redhat.com>
Cc:     Pawan Gupta <pawan.kumar.gupta@linux.intel.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Borislav Petkov <bp@alien8.de>,
        Jonathan Corbet <corbet@lwn.net>,
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        Ingo Molnar <mingo@redhat.com>,
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        Mark Rutland <mark.rutland@arm.com>,
        Alexander Shishkin <alexander.shishkin@linux.intel.com>,
        Jiri Olsa <jolsa@redhat.com>,
        Namhyung Kim <namhyung@kernel.org>, x86@kernel.org,
        "H. Peter Anvin" <hpa@zytor.com>,
        "Paul E. McKenney" <paulmck@kernel.org>,
        Randy Dunlap <rdunlap@infradead.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        "Maciej W. Rozycki" <macro@orcam.me.uk>,
        Viresh Kumar <viresh.kumar@linaro.org>,
        Vlastimil Babka <vbabka@suse.cz>,
        Tony Luck <tony.luck@intel.com>,
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        linux-kernel@vger.kernel.org, linux-perf-users@vger.kernel.org,
        kvm@vger.kernel.org
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On Tue, Jul 6, 2021 at 5:05 PM Paolo Bonzini <pbonzini@redhat.com> wrote:
>
> 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.

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.

>
> 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 either case, we can make new userspace behave well. I'm worried
about existing userspace:

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.


>
> 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
>

-- 
Eduardo