Re: [Xen-devel] [PATCH] vm_event: Record FS_BASE/GS_BASE during events

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 11/02/16 20:00, Razvan Cojocaru wrote:
> On 02/11/2016 09:55 PM, Andrew Cooper wrote:
>> On 11/02/16 19:54, Razvan Cojocaru wrote:
>>> On 02/11/2016 09:51 PM, Tamas K Lengyel wrote:
>>>> While the public vm_event header specifies fs_base/gs_base as registers 
>>>> that
>>>> should be recorded for each event, that hasn't actually been the case. In
>>>> this patch we remedy the issue.
>>>>
>>>> Signed-off-by: Tamas K Lengyel <tlengyel@xxxxxxxxxxx>
>>>> Cc: Razvan Cojocaru <rcojocaru@xxxxxxxxxxxxxxx>
>>>> Cc: Keir Fraser <keir@xxxxxxx>
>>>> Cc: Jan Beulich <jbeulich@xxxxxxxx>
>>>> Cc: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
>>>> ---
>>>>  xen/arch/x86/hvm/event.c | 9 ++++++++-
>>>>  1 file changed, 8 insertions(+), 1 deletion(-)
>>> Fair enough.
>>>
>>> Acked-by: Razvan Cojocaru <rcojocaru@xxxxxxxxxxxxxxx>
>> Oops.
>>
>> Reviewed-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
> This has actually been intentional, in that we've only needed those
> fields for EPT events, and thought that not filling what's not needed
> until it's needed would save a tiny bit of hypervisor processing time.
> They are being filled in only for page fault events at the moment.
>
> I believe it's been discussed at the time. We still don't need those
> coming with the events that use hvm_event_fill_regs(), but if Tamas
> needs them then by all means.

The public header file does suggest that all of vm_event_regs_x86 will
be complete.  Are there any other fields currently missing?

Given the overhead of the vmexit and communication on the vm_event
channel, a few extra cycles reading state is lost in the overhead.

~Andrew

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