Re: [Xen-devel] [PATCH 06/16] SUPPORT.md: Add scalability features

[George Dunlap <george.dunlap@xxxxxxxxxx>]

On 11/21/2017 05:31 PM, Julien Grall wrote:
> Hi George,
> 
> On 11/21/2017 04:43 PM, George Dunlap wrote:
>> On 11/16/2017 03:19 PM, Julien Grall wrote:
>>> On 13/11/17 15:41, George Dunlap wrote:
>>>> Signed-off-by: George Dunlap <george.dunlap@xxxxxxxxxx>
>>>> ---
>>>> CC: Ian Jackson <ian.jackson@xxxxxxxxxx>
>>>> CC: Wei Liu <wei.liu2@xxxxxxxxxx>
>>>> CC: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
>>>> CC: Jan Beulich <jbeulich@xxxxxxxx>
>>>> CC: Stefano Stabellini <sstabellini@xxxxxxxxxx>
>>>> CC: Konrad Wilk <konrad.wilk@xxxxxxxxxx>
>>>> CC: Tim Deegan <tim@xxxxxxx>
>>>> CC: Julien Grall <julien.grall@xxxxxxx>
>>>> ---
>>>>    SUPPORT.md | 21 +++++++++++++++++++++
>>>>    1 file changed, 21 insertions(+)
>>>>
>>>> diff --git a/SUPPORT.md b/SUPPORT.md
>>>> index c884fac7f5..a8c56d13dd 100644
>>>> --- a/SUPPORT.md
>>>> +++ b/SUPPORT.md
>>>> @@ -195,6 +195,27 @@ on embedded platforms.
>>>>      Enables NUMA aware scheduling in Xen
>>>>    +## Scalability
>>>> +
>>>> +### 1GB/2MB super page support
>>>> +
>>>> +    Status, x86 HVM/PVH: : Supported
>>>> +    Status, ARM: Supported
>>>> +
>>>> +NB that this refers to the ability of guests
>>>> +to have higher-level page table entries point directly to memory,
>>>> +improving TLB performance.
>>>> +This is independent of the ARM "page granularity" feature (see below).
>>>
>>> I am not entirely sure about this paragraph for Arm. I understood this
>>> section as support for stage-2 page-table (aka EPT on x86) but the
>>> paragraph lead me to believe to it is for guest.
>>>
>>> The size of super pages of guests will depend on the page granularity
>>> used by itself and the format of the page-table (e.g LPAE vs short
>>> descriptor). We have no control on that.
>>>
>>> What we have control is the size of mapping used for stage-2 page-table.
>>
>> Stepping back from the document for a minute: would it make sense to use
>> "hardware assisted paging" (HAP) for Intel EPT, AMD RVI (previously
>> NPT), and ARM stage-2 pagetables?  HAP was already a general term used
>> to describe the two x86 technologies; and I think the description makes
>> sense, because if we didn't have hardware-assisted stage 2 pagetables
>> we'd need Xen-provided shadow pagetables.
> 
> I think using the term "hardware assisted paging" should be fine to
> refer the 3 technologies.

OK, great.

[snip]

> Short-descriptor is always using 4KB granularity supports 16MB, 1MB, 64KB
> 
> LPAE supports 4KB, 16KB, 64KB granularities. Each of them having
> different size of superpage.

Yes, that's why I started saying "L2 and L3 superpages", to mean
"Superpage entries in L2 or L3 pagetables", instead of 2MiB or 1GiB.
(Let me know if you can think of a better way to describe that.)

>> 3. Whether Xen provides the *interface* for a guest to use L2 or L3
>> superpages (for 4k page granularity, 2MiB or 1GiB respectively) in its
>> own pagetables.  I *think* HAP on x86 provides the interface whenever
>> the underlying hardware does.  I assume it's the same for ARM?  In the
>> case of shadow mode, we only provide the interface for 2MiB pagetables.
> 
> See above. We have no way to control that in the guest.

We don't control whether the guest uses *any* features.  Should we not
mention PV disks or SMMUv2 or whatever because we don't know if the
guest will use them?

Of course not.  This document describes whether the guest *has the
features available to use*, either provided by the hardware or emulated
by Xen.

It sounds like you may not have ever thought about whether an ARM guest
has L2 or L3 superpages available, because it's always had all of them;
but it's different on x86.

[snip]

>> 2. Whether Xen uses superpage mappings for HAP.  Xen uses this on x86
>> when hardware support is -- I take it Xen does this on ARM as well?
>
> The size of superpages supported will depend on the page-table format
> (short-descriptor vs LPAE) and the granularity used.
>
> Supersection (16MB) for short-descriptor is optional but mandatory when
> the processor support LPAE. LPAE is mandatory with virtualization. So
> all size of superpages are supported.
>
> Note that stage-2 page-tables can only use LPAE page-table.
>
> I would also rather avoid to mention any superpage size for Arm in
> SUPPORT.MD as there are a lot.

So it sounds like basically everything supported on native was supported
in virtualization (and under Xen) from the start, so it's probably less
important to mention.  But since we *will* need to do that for x86, we
probably need to say *something* in case people want to know.

Let me see what I can come up with.

 -George

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