Re: [Xen-devel] [PATCH] vmx: fix debugctl handling

["Jan Beulich" <jbeulich@xxxxxxxxxx>]

>>> "Haitao Shan" <maillists.shan@xxxxxxxxx> 17.03.08 14:38 >>>
>Yes, I agree. Once allowing guest to modify this MSR, you have to implement
>some save/load functions.
>I am wondering what you are trying to enable? As far as I know, DS area

Yes, I'm considering adding support for DS, since Linux 2.6.25-rc briefly
had support for this (it's now disabled again), so it's likely that pretty
soon the functionality in the kernel will be there. Hence the hypervisor
ought to support it.

>(both PEBS and BTS) is not working properly now. You should assume that any
>events which will trigger DS write might happen right at the moment that CPU
>is in VMExit/VMEntry. Only using HW to switch related MSRs is not enough.

That would be very bad - it would mean that you cannot reliably
virtualize this. But - are you sure here? For PEBS, I could see this to
be true (although it should be properly dealt with in hardware), but
BTS shouldn't have any activity during vmexit/vmentry as there are
no branches during that time.

Jan


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