Re: [Xen-devel] [PATCH] vmx/monitor: CPUID events

[Tamas K Lengyel <tamas.lengyel@xxxxxxxxxxxx>]

On Fri, Jul 8, 2016 at 3:33 AM, Andrew Cooper <andrew.cooper3@xxxxxxxxxx> wrote:
> On 08/07/16 03:31, Tamas K Lengyel wrote:
>> This patch implements sending notification to a monitor subscriber when an
>> x86/vmx guest executes the CPUID instruction.
>>
>> Signed-off-by: Tamas K Lengyel <tamas.lengyel@xxxxxxxxxxxx>
>
> Is it wise having an on/off control without any further filtering?  (I
> suppose that it is at least a fine first start).

What type of extra filtering do you have in mind?

>
> cpuid is usually the serialising instruction used with rdtsc for timing
> loops.  This is bad enough in VMs because of the VMExit, but becomes
> even worse if there is a monitor delay as well.

Yes, going the extra route of sending a monitor event out will add to
that delay (how much delay will depend on the subscriber and what it
decides to do with the event). Wouldn't we be able to mask some of
that with tsc offsetting though?

Thanks,
Tamas

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