Re: [Xen-devel] [PATCH RFC v2 00/12] xen/x86: use per-vcpu stacks for 64 bit pv domains

[Juergen Gross <jgross@xxxxxxxx>]

On 23/01/18 10:31, Jan Beulich wrote:
>>>> On 23.01.18 at 10:24, <jgross@xxxxxxxx> wrote:
>> On 23/01/18 09:53, Jan Beulich wrote:
>>>>>> On 23.01.18 at 07:34, <jgross@xxxxxxxx> wrote:
>>>> On 22/01/18 19:39, Andrew Cooper wrote:
>>>>> One of my concerns is that this patch series moves further away from the
>>>>> secondary goal of my KAISER series, which was to have the IDT and GDT
>>>>> mapped at the same linear addresses on every CPU so a) SIDT/SGDT don't
>>>>> leak which CPU you're currently scheduled on into PV guests and b) the
>>>>> context switch code can drop a load of its slow instructions like LGDT
>>>>> and the VMWRITEs to update the VMCS.
>>>>
>>>> The GDT address of a PV vcpu is depending on vcpu_id only. I don't
>>>> see why the IDT can't be mapped to the same address on each cpu with
>>>> my approach.
>>>
>>> You're not introducing a per-CPU range in the page tables afaics
>>> (again from overview and titles only), yet with the IDT needing
>>> to be per-CPU you'd also need a per-CPU range to map it to if
>>> you want to avoid the LIDT as well as exposing what CPU you're
>>> on (same goes for the GDT and the respective avoidance of LGDT
>>> afaict).
>>
>> After a quick look I don't see why a Meltdown mitigation can't use
>> the same IDT for all cpus: the only reason I could find for having
>> per-cpu IDTs seems to be in SVM code, so it seems to be AMD specific.
>> And AMD won't need XPTI at all.
> 
> Isn't your RFC series allowing XPTI to be enabled even on AMD?

Yes, you are right. This might either want to be revisited or the
address space to be activated for SVM domains could map an IDT with
IST related traps removed.

>> The GDT of pv domains is already in the per-domain region even without
>> my patches, so I don't have to change anything regarding usage of LGDT.
> 
> Andrew's point was that eliminating the LGDT is a secondary goal.

With per-cpu mappings this is surely an obvious optimization. In the
end the overall performance should be taken as base for a decision.
His main point was avoiding exposing data like the physical cpu number
and this doesn't apply here, as the GDT is per vcpu in my case.


Juergen

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