From: Paolo Bonzini <pbonzini@redhat.com>
To: Andy Lutomirski <luto@amacapital.net>,
kvm list <kvm@vger.kernel.org>,
"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
X86 ML <x86@kernel.org>, "H. Peter Anvin" <hpa@zytor.com>,
Borislav Petkov <bpetkov@suse.de>
Subject: Re: RFC: Getting rid of LTR in VMX
Date: Sat, 18 Feb 2017 16:19:17 +0100 [thread overview]
Message-ID: <1edbf913-a199-264f-c00e-8b07dd5e5ace@redhat.com> (raw)
In-Reply-To: <CALCETrVnHc1_M0_zEL=EBvMNGBj-ZqzvhrGBw9jKJ6HmL-qKDg@mail.gmail.com>
On 18/02/2017 04:29, Andy Lutomirski wrote:
> There's no code here because the patch is trivial, but I want to run
> the idea by you all first to see if there are any issues.
>
> VMX is silly and forces the TSS limit to the minimum on VM exits. KVM
> wastes lots of cycles bumping it back up to accomodate the io bitmap.
>
> I propose that we rework this. Add a percpu variable that indicates
> whether the TSS limit needs to be refreshed. On task switch, if the
> new task has TIF_IO_BITMAP set, then check that flag and, if set,
> refresh TR and clear the flag. On VMX exit, set the flag.
That's nice. An alternative is to just use the user return notifier,
which does not need any global change. This is even simpler and can be
used to benchmark.
> The TSS limit is (phew!) invisible to userspace, so we don't have ABI
> issues to worry about here. We also shouldn't have security issues
> because a too-low TSS limit just results in unprivileged IO operations
> generating #GP, which is exactly what we want.
>
> What do you all think? I expect a speedup of a couple hundred cycles
> on each VM exit.
>
next prev parent reply other threads:[~2017-02-18 15:19 UTC|newest]
Thread overview: 8+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-02-18 3:29 RFC: Getting rid of LTR in VMX Andy Lutomirski
2017-02-18 15:19 ` Paolo Bonzini [this message]
2017-02-20 11:05 ` Paolo Bonzini
2017-02-20 16:46 ` Andy Lutomirski
2017-02-20 16:51 ` Paolo Bonzini
2017-02-20 16:55 ` Andy Lutomirski
2017-02-20 22:02 ` Paolo Bonzini
2017-02-20 22:23 ` hpa
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