From: Vitaly Kuznetsov <vkuznets@redhat.com>
To: Marcelo Tosatti <mtosatti@redhat.com>
Cc: kvm@vger.kernel.org, Sean Christopherson <seanjc@google.com>,
Wanpeng Li <wanpengli@tencent.com>,
Jim Mattson <jmattson@google.com>,
Paolo Bonzini <pbonzini@redhat.com>
Subject: Re: [PATCH v2 2/4] KVM: x86: hyper-v: Prevent using not-yet-updated TSC page by secondary CPUs
Date: Fri, 19 Mar 2021 10:29:44 +0100 [thread overview]
Message-ID: <87tup75w53.fsf@vitty.brq.redhat.com> (raw)
In-Reply-To: <20210318183042.GA42884@fuller.cnet>
Marcelo Tosatti <mtosatti@redhat.com> writes:
> On Thu, Mar 18, 2021 at 07:05:49PM +0100, Paolo Bonzini wrote:
>> On 18/03/21 19:04, Marcelo Tosatti wrote:
>> > >
>> > > Not clear why this is necessary, if the choice was to not touch TSC page
>> > > at all, when invariant TSC is supported on the host...
>> >
>> > s/invariant TSC/TSC scaling/
>> >
>> > > Ah, OK, this is not for the migration with iTSC on destination case,
>> > > but any call to kvm_gen_update_masterclock, correct?
>>
>> Yes, any update can be racy.
>>
>> Paolo
>
> Which makes an unrelated KVM_REQ_MASTERCLOCK_UPDATE -> kvm_gen_update_masterclock
> sequence to inadvertedly reduce performance a possibility, unless i am
> missing something.
>
> Ah, OK, it should be enabled again at KVM_REQ_CLOCK_UPDATE.
Right,
this patch is loosely related to the original problem, it's just
something I've noticed while debugging. Unlike kvmclock, which is
per-cpu, TSC page is global so only vCPU0 updates it in
kvm_guest_time_update() but the potential problem is: some other vCPU
may enter the guest before we manage to update the page on vCPU0 and
this is racy. Here, we just follow TLFS: invalidate TSC page while we're
updating it (on KVM_REQ_MASTERCLOCK_UPDATE).
PATCH4, which avoids updating TSC page when re-enlightenment is enabled,
also disables this invalidation. This is OK as we're not going to update
it.
--
Vitaly
next prev parent reply other threads:[~2021-03-19 9:30 UTC|newest]
Thread overview: 22+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-03-16 14:37 [PATCH v2 0/4] KVM: x86: hyper-v: TSC page fixes Vitaly Kuznetsov
2021-03-16 14:37 ` [PATCH v2 1/4] KVM: x86: hyper-v: Limit guest to writing zero to HV_X64_MSR_TSC_EMULATION_STATUS Vitaly Kuznetsov
2021-03-16 14:37 ` [PATCH v2 2/4] KVM: x86: hyper-v: Prevent using not-yet-updated TSC page by secondary CPUs Vitaly Kuznetsov
2021-03-18 17:02 ` Marcelo Tosatti
2021-03-18 18:04 ` Marcelo Tosatti
2021-03-18 18:05 ` Paolo Bonzini
2021-03-18 18:30 ` Marcelo Tosatti
2021-03-19 9:29 ` Vitaly Kuznetsov [this message]
2021-03-16 14:37 ` [PATCH v2 3/4] KVM: x86: hyper-v: Track Hyper-V TSC page status Vitaly Kuznetsov
2021-03-17 8:07 ` Paolo Bonzini
2021-03-17 11:19 ` [PATCH v2 5/4] KVM: x86: hyper-v: Briefly document enum hv_tsc_page_status Vitaly Kuznetsov
2021-03-16 14:37 ` [PATCH v2 4/4] KVM: x86: hyper-v: Don't touch TSC page values when guest opted for re-enlightenment Vitaly Kuznetsov
2021-03-18 14:09 ` [PATCH v2 6/4] selftests: kvm: Add basic Hyper-V clocksources tests Vitaly Kuznetsov
2021-03-18 14:26 ` Paolo Bonzini
2021-03-18 14:52 ` Vitaly Kuznetsov
2021-03-18 15:01 ` Paolo Bonzini
2021-03-18 15:23 ` Vitaly Kuznetsov
2021-03-18 15:27 ` Paolo Bonzini
2021-03-18 16:57 ` Marcelo Tosatti
2021-03-18 17:50 ` Paolo Bonzini
2021-03-18 17:55 ` Marcelo Tosatti
2021-03-19 9:35 ` Vitaly Kuznetsov
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=87tup75w53.fsf@vitty.brq.redhat.com \
--to=vkuznets@redhat.com \
--cc=jmattson@google.com \
--cc=kvm@vger.kernel.org \
--cc=mtosatti@redhat.com \
--cc=pbonzini@redhat.com \
--cc=seanjc@google.com \
--cc=wanpengli@tencent.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).