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Re: [PATCH] target/i386: introduce CPU property to work around Windows r


From: Paolo Bonzini
Subject: Re: [PATCH] target/i386: introduce CPU property to work around Windows reset bug
Date: Thu, 24 Mar 2022 10:42:22 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101 Thunderbird/91.4.0

On 3/24/22 10:15, Daniel P. Berrangé wrote:
On Thu, Mar 24, 2022 at 09:13:12AM +0000, Daniel P. Berrangé wrote:
On Thu, Mar 24, 2022 at 09:23:46AM +0100, Paolo Bonzini wrote:
Some versions of Windows hang on reboot if their TSC value is greater
than 2^54.  The calibration of the Hyper-V reference time overflows
and fails; as a result the processors' clock sources are out of sync.
As a workaround, reset the TSC to a small value.  Do not do this
unconditionally and require a special property to be set.

What's the problem with doing it unconditionally ?

Requiring this special niche property means that it'll have to be
enabled by management apps. Most will never learn it exists, and
of those that do, many will take years to get this enabled and
into usage by users, and many won't even bother.

IMHO, this is the kind of situation where we need the fix to be
enabled by default, or we might as well not bother.

Sigh, hit send too soon. I see the property is actually turned
on in the defaults below, so effectively it will always be on
unconditionally as no one will bother to add support for turning
it off.

Well, I have a patch to turn it on/off in Libvirt and I also planned to leave it off by default in RHEL patch updates (I'm not tying it to the machine type because it's not a guest ABI change).

I am myself conflicted on whether to leave it on or off in QEMU. For example you could use the TSC to measure how long the VM has been up, but this patch makes that not work anymore. Considering that the bug requires literally 2-3 months of VM uptime to manifest itself, it might be better to set up the property in libosinfo and only for Windows guests.

Also, since it is a bug in Windows, it will hopefully be fixed sooner or later.

Paolo



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