Re: [Xen-devel] [PATCH v7 1/3] x86/tlb: introduce a flush HVM ASIDs flag

[Jan Beulich <jbeulich@xxxxxxxx>]

On 20.03.2020 10:12, Julien Grall wrote:
> On 20/03/2020 09:01, Roger Pau Monné wrote:
>> On Fri, Mar 20, 2020 at 08:21:19AM +0100, Jan Beulich wrote:
>>> On 19.03.2020 20:07, Julien Grall wrote:
>>>> On 19/03/2020 18:43, Roger Pau Monné wrote:
>>>>> On Thu, Mar 19, 2020 at 06:07:44PM +0000, Julien Grall wrote:
>>>>>> On 19/03/2020 17:38, Roger Pau Monné wrote:
>>>>>>> On Thu, Mar 19, 2020 at 04:21:23PM +0000, Julien Grall wrote:
>>>>>>>    >> Why can't you keep flush_tlb_mask() here?
>>>>>>>
>>>>>>> Because filtered_flush_tlb_mask is used in populate_physmap, and
>>>>>>> changes to the phymap require an ASID flush on AMD hardware.
>>>>>>
>>>>>> I am afraid this does not yet explain me why flush_tlb_mask() could not 
>>>>>> be
>>>>>> updated so it flush the ASID on AMD hardware.
>>>>>
>>>>> Current behavior previous to this patch is to flush the ASIDs on
>>>>> every TLB flush.
>>>>>
>>>>> flush_tlb_mask is too widely used on x86 in places where there's no
>>>>> need to flush the ASIDs. This prevents using assisted flushes (by L0)
>>>>> when running nested, since those assisted flushes performed by L0
>>>>> don't flush the L2 guests TLBs.
>>>>>
>>>>> I could keep current behavior and leave flush_tlb_mask also flushing the
>>>>> ASIDs, but that seems wrong as the function doesn't have anything in
>>>>> it's name that suggests it also flushes the in-guest TLBs for HVM.
>>>>
>>>> I agree the name is confusing, I had to look at the implementation to 
>>>> understand what it does.
>>>>
>>>> How about renaming (or introducing) the function to 
>>>> flush_tlb_all_guests_mask() or flush_tlb_all_guests_cpumask()) ?
>>>
>>> And this would then flush _only_ guest TLBs?
>>
>> No, I think from Julien's proposal (if I understood it correctly)
>> flush_tlb_all_guests_mask would do what flush_tlb_mask currently does
>> previous to this patch (flush Xen's TLBs + HVM ASIDs).
> 
> It looks like there might be confusion on what "guest TLBs" means. In my view 
> this means any TLBs associated directly or indirectly with the guest. On Arm, 
> this would be nuke:
>    - guest virtual address -> guest physical address TLB entry
>    - guest physical address -> host physical address TLB entry
>    - guest virtual address -> host physical address TLB entry
> 
> I would assume you want something similar on x86, right?

I don't think we'd want the middle of the three items you list,
but I also don't see how this would be relevant here - flushing
that is a p2m operation, not one affecting in-guest translations.

Jan

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