From: David Woodhouse <dwmw2@infradead.org>
To: Thomas Gleixner <tglx@linutronix.de>
Cc: karahmed@amazon.de, x86@kernel.org, kvm@vger.kernel.org,
torvalds@linux-foundation.org, pbonzini@redhat.com,
linux-kernel@vger.kernel.org, bp@alien8.de, peterz@infradead.org,
jmattson@google.com, rkrcmar@redhat.com,
arjan.van.de.ven@intel.com, dave.hansen@intel.com,
mingo@kernel.org
Subject: Re: [PATCH v3 2/4] x86/speculation: Support "Enhanced IBRS" on future CPUs
Date: Tue, 20 Feb 2018 11:22:05 +0000 [thread overview]
Message-ID: <1519125725.7876.117.camel@infradead.org> (raw)
In-Reply-To: <alpine.DEB.2.21.1802201141160.24268@nanos.tec.linutronix.de>
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On Tue, 2018-02-20 at 11:42 +0100, Thomas Gleixner wrote:
>
> > > However, Paolo is very insistent that taking the trap every time is
> > > actually a lot *slower* than really frobbing IBRS on certain
> > > microarchitectures, so my hand-waving "pfft, what did they expect?" is
> > > not acceptable.
> > >
> > > Which I think puts us back to the "throwing the toys out of the pram"
>
> There are no more toys in the pram. I threw them all out weeks ago ...
One option is to take the patch as-is¹ with the trap on every access.
As soon as Intel define that 'IBRS_ALL_AND_THE_BIT_IS_A_NOOP' bit in
MSR_IA32_ARCH_CAPABILITIES, *then* we can expose it to guests directly
again just as we do at the moment.
That way, the slowdown that Paolo is concerned about is limited to a
small set of current CPUs on which we're mostly unlikely to care too
much about KVM anyway.
¹ as-is, except I really will go and turn the IA32_ARCH_CAPABILITIES
bits into scattered cpufeatures before I repost it.
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next prev parent reply other threads:[~2018-02-20 11:22 UTC|newest]
Thread overview: 20+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-02-19 10:50 [PATCH v3 0/4] Speculation control improvements David Woodhouse
2018-02-19 10:50 ` [PATCH v3 1/4] x86/speculation: Use IBRS if available before calling into firmware David Woodhouse
2018-02-20 7:44 ` Thomas Gleixner
2018-02-20 10:29 ` [tip:x86/pti] " tip-bot for David Woodhouse
2018-02-19 10:50 ` [PATCH v3 2/4] x86/speculation: Support "Enhanced IBRS" on future CPUs David Woodhouse
2018-02-20 8:31 ` Thomas Gleixner
2018-02-20 8:53 ` David Woodhouse
2018-02-20 10:37 ` Thomas Gleixner
2018-02-20 10:42 ` Thomas Gleixner
2018-02-20 11:22 ` David Woodhouse [this message]
2018-02-20 11:28 ` Paolo Bonzini
2018-02-26 19:55 ` Thomas Gleixner
2018-02-20 11:26 ` Paolo Bonzini
2018-02-19 10:50 ` [PATCH v3 3/4] Revert "x86/retpoline: Simplify vmexit_fill_RSB()" David Woodhouse
2018-02-20 8:35 ` Thomas Gleixner
2018-02-20 10:28 ` [tip:x86/pti] " tip-bot for David Woodhouse
2018-02-19 10:50 ` [PATCH v3 4/4] x86/retpoline: Support retpoline build with Clang David Woodhouse
2018-02-20 8:36 ` Thomas Gleixner
2018-02-20 8:45 ` David Woodhouse
2018-02-20 10:29 ` [tip:x86/pti] x86/retpoline: Support retpoline builds " tip-bot for David Woodhouse
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