linux-kernel - Re: [PATCH] KVM: x86: Don't snapshot "max" TSC if host TSC is constant

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <654a4070a547cf6a5dd4b2b42c41930a7900644a.camel@infradead.org>
Date:   Fri, 25 Feb 2022 16:34:00 +0000
From:   David Woodhouse <dwmw2@...radead.org>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Suleiman Souhlal <suleiman@...gle.com>,
        Anton Romanov <romanton@...gle.com>
Subject: Re: [PATCH] KVM: x86: Don't snapshot "max" TSC if host TSC is
 constant

On Fri, 2022-02-25 at 16:21 +0000, Sean Christopherson wrote:
> > I also have questions about the use case for the above patch.... if
> > this is a clean boot and you're just starting to host guests, surely we
> > can wait for the time it takes for the TSC synchronization to complete?
> 
> KVM is built into the kernel in their case, the vmx_init() => kvm_init() gets
> automatically called during boot.  The VMs aren't started until well after
> synchronization has completed, but KVM has already snapshotted the "bad" value.


Gotcha.

So even when we put my patch in front, to snapshot a value into
kvm->arch.default_tsc_khz, that's happening later at VM creation time
so should also be snapshotting the *good* value.

And at least if it snapshots the bad value, all the vCPUs will be
*consistent*.

Download attachment "smime.p7s" of type "application/pkcs7-signature" (5965 bytes)