lkml.org 
[lkml]   [2020]   [Dec]   [10]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
From
SubjectRe: [PATCH v2 1/3] KVM: x86: implement KVM_{GET|SET}_TSC_STATE
Date
On 07/12/20 18:41, Thomas Gleixner wrote:
> Right this happens still occasionally, but for quite some time this is
> 100% firmware sillyness and not a fundamental property of the hardware
> anymore.

It's still a fundamental property of old hardware. Last time I tried to
kill support for processors earlier than Core 2, I had to revert it.
That's older than Nehalem.

>> We try to catch such situation in KVM instead of blowing up but
>> this may still result in subtle bugs I believe. Maybe we would be better
>> off killing all VMs in case TSC ever gets unsynced (by default).
>
> I just ran a guest on an old machine with unsynchronized TSCs and was
> able to observe clock monotonic going backwards between two threads
> pinned on two vCPUs, which _is_ bad. Getting unsynced clocks reliably
> under control is extremly hard.

Using kvmclock? (Half serious: perhaps a good reason to have per-vCPU
offsets is to be able to test what happens with unsynchronized TSCs...).

Paolo

\
 
 \ /
  Last update: 2020-12-10 12:50    [W:0.128 / U:0.944 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site