From: Josh Poimboeuf <jpoimboe@redhat.com>
To: Thomas Gleixner <tglx@linutronix.de>
Cc: Paolo Bonzini <pbonzini@redhat.com>,
"Lendacky, Thomas" <Thomas.Lendacky@amd.com>,
"x86@kernel.org" <x86@kernel.org>,
"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
Andrew Cooper <andrew.cooper3@citrix.com>,
Pu Wen <puwen@hygon.cn>, Borislav Petkov <bp@alien8.de>
Subject: Re: [RFC PATCH] x86: Remove X86_FEATURE_MFENCE_RDTSC
Date: Thu, 11 Jul 2019 12:45:07 -0500 [thread overview]
Message-ID: <20190711174507.wrwfxohzutfsgbgl@treble> (raw)
In-Reply-To: <alpine.DEB.2.21.1907101333250.1758@nanos.tec.linutronix.de>
On Wed, Jul 10, 2019 at 01:33:48PM +0200, Thomas Gleixner wrote:
> On Wed, 10 Jul 2019, Paolo Bonzini wrote:
> > On 08/07/19 21:32, Lendacky, Thomas wrote:
> > >> AMD and Intel both have serializing lfence (X86_FEATURE_LFENCE_RDTSC).
> > >> They've both had it for a long time, and AMD has had it enabled in Linux
> > >> since Spectre v1 was announced.
> > >>
> > >> Back then, there was a proposal to remove the serializing mfence feature
> > >> bit (X86_FEATURE_MFENCE_RDTSC), since both AMD and Intel have
> > >> serializing lfence. At the time, it was (ahem) speculated that some
> > >> hypervisors might not yet support its removal, so it remained for the
> > >> time being.
> > >>
> > >> Now a year-and-a-half later, it should be safe to remove.
> > >
> > > I vaguely remember a concern from a migration point of view, maybe? Adding
> > > Paolo to see if he has any concerns.
> >
> > It would be a problem to remove the conditional "if
> > (boot_cpu_has(X86_FEATURE_LFENCE_RDTSC))" from svm_get_msr_feature. But
> > removing support for X86_FEATURE_MFENCE_RDTSC essentially amounts to
> > removing support for hypervisors that haven't been updated pre-Spectre.
> > That's fair enough, I think.
>
> Yes, they have other more interesting problems :)
Great. Anyone care to give an ACK? :-)
--
Josh
next prev parent reply other threads:[~2019-07-11 17:45 UTC|newest]
Thread overview: 6+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-07-04 15:46 [RFC PATCH] x86: Remove X86_FEATURE_MFENCE_RDTSC Josh Poimboeuf
2019-07-08 19:32 ` Lendacky, Thomas
2019-07-10 6:20 ` Paolo Bonzini
2019-07-10 11:33 ` Thomas Gleixner
2019-07-11 17:45 ` Josh Poimboeuf [this message]
2019-07-22 10:04 ` [tip:x86/cpu] " tip-bot for Josh Poimboeuf
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