All of lore.kernel.org
 help / color / mirror / Atom feed
From: "Radim Krčmář" <rkrcmar@redhat.com>
To: Paolo Bonzini <pbonzini@redhat.com>
Cc: Wanpeng Li <kernellwp@gmail.com>,
	linux-kernel@vger.kernel.org, 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: Wed, 14 Sep 2016 14:03:25 +0200	[thread overview]
Message-ID: <20160914120325.GG15680@potion> (raw)
In-Reply-To: <9c0023da-be74-2fc2-0fd9-6bdfb62862b3@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?

> 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).

  reply	other threads:[~2016-09-14 12:03 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 [this message]
2016-09-15  1:19     ` Wanpeng Li
2016-09-15  6:29       ` Paolo Bonzini
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=20160914120325.GG15680@potion \
    --to=rkrcmar@redhat.com \
    --cc=fanwenyi0529@gmail.com \
    --cc=kernellwp@gmail.com \
    --cc=kvm@vger.kernel.org \
    --cc=linux-kernel@vger.kernel.org \
    --cc=pbonzini@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 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.