From: Paolo Bonzini <pbonzini@redhat.com>
To: "Wanpeng Li" <kernellwp@gmail.com>, "Radim Krčmář" <rkrcmar@redhat.com>
Cc: "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
kvm <kvm@vger.kernel.org>, Wanpeng Li <wanpeng.li@hotmail.com>,
Wincy Van <fanwenyi0529@gmail.com>,
Yang Zhang <yang.zhang.wz@gmail.com>
Subject: Re: [PATCH] KVM: VMX: Enable MSR-BASED TPR shadow even if w/o APICv
Date: Thu, 15 Sep 2016 08:29:38 +0200 [thread overview]
Message-ID: <264e471d-5ca7-f7fb-b34c-3cc9c68a29da@redhat.com> (raw)
In-Reply-To: <CANRm+CxO1UXW9BZdt7mDjtyAhCOKKg4WO5w3gp1RhSE=pMOHHw@mail.gmail.com>
On 15/09/2016 03:19, Wanpeng Li wrote:
> 2016-09-14 20:03 GMT+08:00 Radim Krčmář <rkrcmar@redhat.com>:
>> 2016-09-14 11:40+0200, Paolo Bonzini:
>>> On 14/09/2016 09:58, Wanpeng Li wrote:
>>>> From: Wanpeng Li <wanpeng.li@hotmail.com>
>>>>
>>>> I observed that kvmvapic(to optimize flexpriority=N or AMD) is used
>>>> to boost TPR access when testing kvm-unit-test/eventinj.flat tpr case
>>>> on my haswell desktop (w/ flexpriority, w/o APICv). Commit (8d14695f9542
>>>> x86, apicv: add virtual x2apic support) disable virtual x2apic mode
>>>> completely if w/o APICv, and the author also told me that windows guest
>>>> can't enter into x2apic mode when he developed the APICv feature several
>>>> years ago. However, it is not truth currently, Interrupt Remapping and
>>>> vIOMMU is added to qemu and the developers from Intel test windows 8 can
>>>> work in x2apic mode w/ Interrupt Remapping enabled recently.
>>>>
>>>> This patch enables TPR shadow for virtual x2apic mode to boost
>>>> windows guest in x2apic mode even if w/o APICv.
>>>>
>>>> Can pass the kvm-unit-test.
>>>
>>> Ok, now I see what you meant; this actually makes sense. I don't expect
>>> much speedup though, because Linux doesn't touch the TPR and Windows is
>>> likely going to use the Hyper-V APIC MSRs when APICv is disabled. For
>>> this reason I'm not sure if the patch is useful in practice.
>>
>> I agree with Paolo on the use case -- what configurations benefit from
>> this change?
>
> Old windows guest w/o Hyper-V synthetic interrupt support.
... but with Hyper-V synthetic interrupt support enabled in the QEMU
command line. Right?
Paolo
>>
>>> To test this patch, you have to run kvm-unit-tests with Hyper-V
>>> synthetic interrupt enabled. Did you do this?
>>
>> The patch is buggy. MSR bitmaps are global and we'd have a CVE if one
>> guests used synic (=> disabled apicv) and one didn't.
>> You'd want a new set of bitmaps and assign them in vmx_set_msr_bitmap()
>> (or completely rewrite our management).
>
next prev parent reply other threads:[~2016-09-15 6:29 UTC|newest]
Thread overview: 19+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-09-14 7:58 [PATCH] KVM: VMX: Enable MSR-BASED TPR shadow even if w/o APICv Wanpeng Li
2016-09-14 9:40 ` Paolo Bonzini
2016-09-14 12:03 ` Radim Krčmář
2016-09-15 1:19 ` Wanpeng Li
2016-09-15 6:29 ` Paolo Bonzini [this message]
2016-09-15 6:40 ` Wanpeng Li
2016-09-15 7:05 ` Wanpeng Li
2016-09-15 15:58 ` Radim Krčmář
2016-09-18 6:53 ` Wanpeng Li
2016-09-18 6:55 ` Wanpeng Li
2016-09-19 13:44 ` Radim Krčmář
2016-09-20 0:18 ` Wanpeng Li
2016-09-22 6:48 ` Wanpeng Li
2016-09-15 0:07 ` Wanpeng Li
2016-09-15 0:17 ` Wanpeng Li
2016-09-15 4:08 ` Mika Penttilä
2016-09-15 4:25 ` Wanpeng Li
2016-09-15 4:46 ` Mika Penttilä
2016-09-15 6:30 ` Paolo Bonzini
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=264e471d-5ca7-f7fb-b34c-3cc9c68a29da@redhat.com \
--to=pbonzini@redhat.com \
--cc=fanwenyi0529@gmail.com \
--cc=kernellwp@gmail.com \
--cc=kvm@vger.kernel.org \
--cc=linux-kernel@vger.kernel.org \
--cc=rkrcmar@redhat.com \
--cc=wanpeng.li@hotmail.com \
--cc=yang.zhang.wz@gmail.com \
/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 a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).