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

[Paolo Bonzini <pbonzini@redhat.com>]

On 3/24/22 12:03, Daniel P. Berrangé wrote:
> 
>    "This only applies to virtual machine hardware version 10 as Windows
>     resets the TSC on all CPUs on virtual machines with older hardware
>     versions (which do not support hypervisor.cpuid.v2)."
> 
> do you know what they mean when they refer to 'hypervisor.cpuid.v2'
> here ? I wonder if it gives any hints as to a root cause that could
> be fixed ?

The difference between hardware versions probably is that older versions 
do not support Hyper-V enlightenments.  The bug does not happen if 
Windows uses the RTC for timekeeping, which is consistent with the 
description above.

> This hardware version 10 is well old - their current hardware version
> is 19, so it seems to show the implemented some built-in fix in newer
> hardware versions (their equiv of machine types). The vmware setting
> dates from 2013, and if I read that kbase correctly isn't needed on
> their modern hardware versions. Or maybe monitor_control.enable_softResetClearTSC
> became the default in newer hardware versions ?

Yeah, maybe it became the default but they didn't want to change 
previously-released hardware versions.  So that would be basically 
treating it as an ABI change, and only enable it in 7.0 machine types.

That said, the VMware kbase does paint a slightly different picture.  It 
implies that starting with hardware version 11 rebooting Windows is done 
through a hard reset instead of INIT.  I'm not sure how that would be 
done, but in the meanwhile our fix should take care of do_cpu_init as well.

Paolo