From: Andy Lutomirski <luto@amacapital.net>
To: Paolo Bonzini <pbonzini@redhat.com>
Cc: 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: Mon, 20 Feb 2017 08:46:14 -0800 [thread overview]
Message-ID: <CALCETrUU5Co-gKzsu9ZhVGOBSikLzxm7_6ZoAYVYU5E6SSnV1g@mail.gmail.com> (raw)
In-Reply-To: <38d613dc-5b46-1f4f-04d1-53c01932e6d6@redhat.com>
On Mon, Feb 20, 2017 at 3:05 AM, Paolo Bonzini <pbonzini@redhat.com> wrote:
>
>
> 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.
>
> Actually looked at the code now...
>
> reload_tss is only invoked for userspace exits, so it is a nice-to-have
> but it wouldn't show on most workloads. Still it does save 150-200
> clock cycles to remove it (I just commented out reload_tss() from
> __vmx_load_host_state to test).
That's for anything involving userspace or preemption, right?
>
> Another 100-150 could be saved if we could just use rdgsbase/wrgsbase,
> instead of rdmsr/wrmsr, to read and write the kernel GS. Super hacky
> patch after sig.
I have a Real Patch Series (tm) to do that, but it has a couple of
unresolved corner cases so far.
That being said, vmx_save_host_state() is, um, poorly optimized. I'll
try to find some time to fix the obvious things. Meanwhile, I'll send
real patches for TR.
> + cr4_set_bits(X86_CR4_FSGSBASE);
Nice root hole :-p
--Andy
next prev parent reply other threads:[~2017-02-20 16:46 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
2017-02-20 11:05 ` Paolo Bonzini
2017-02-20 16:46 ` Andy Lutomirski [this message]
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
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=CALCETrUU5Co-gKzsu9ZhVGOBSikLzxm7_6ZoAYVYU5E6SSnV1g@mail.gmail.com \
--to=luto@amacapital.net \
--cc=bpetkov@suse.de \
--cc=hpa@zytor.com \
--cc=kvm@vger.kernel.org \
--cc=linux-kernel@vger.kernel.org \
--cc=pbonzini@redhat.com \
--cc=x86@kernel.org \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is an external index of several public inboxes,
see mirroring instructions on how to clone and mirror
all data and code used by this external index.