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

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 08/07/16 16:44, Tamas K Lengyel wrote:
> 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?

Not sure.  What are you intending to use this facility for?

Given that the hypervisor is already in complete control of what a guest
gets to see via cpuid, mutating the results via the monitor framework
doesn't seem like a useful thing to do.

>
>> 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?

I am going to go out on a limb and say that that is a very large can of
worms which you don't want to open.

The problem is not that time skews from the point of view of the guest,
but that the timing loop with a fixed number of iterations takes
proportionally longer.

~Andrew

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