Re: [RFC] Support of non-indirect grant backend on 64KB guest

[David Vrabel <david.vrabel@citrix.com>]

On 20/08/15 09:31, Roger Pau Monné wrote:
> El 20/08/15 a les 1.44, Stefano Stabellini ha escrit:
>> On Wed, 19 Aug 2015, Roger Pau Monné wrote:
>>> My opinion is that we have already merged quite a lot of this mess in
>>> order to support guests with different page sizes. And in this case, the
>>> addition of code can be done to a userspace component, which is much
>>> less risky than adding it to blkfront, also taking into account that
>>> it's a general improvement for Qdisk that other arches can also leverage.
>>>
>>> So in one hand you are adding code to a kernel component, that makes the
>>> code much more messy and can only be leveraged by ARM. On the other
>>> hand, you can add code a user-space backend, and that code is also
>>> beneficial for other arches. IMHO, the decision is quite clear.
>>
>> 64K pages not working is entirely a Linux problem, not a Xen problem.
>> Xen uses 4K pages as usual and exports the same 4K based hypercall
>> interface as usual. That needs to work, no matter what the guest decides
>> to put in its own pagetables.
>>
>> I remind everybody that Xen interfaces on ARM and ARM64 are fully
>> maintained for backward compatibility. Xen is not forcing Linux to use
>> 64K pages, that's entirely a Linux decision. The issue has nothing to do
>> with Xen.
>>
>> The bug here is that Linux has broken 64K pages support and that should
>> be fixed. I don't think is reasonable to make changes to the Xen ABIs
>> just to accommodate the brokenness of one guest kernel in a particular
>> configuration.
> 
> Is it a change to the ABI to mandate indirect-descriptors support in
> order to run arm64 with 64KB guests?

No (given that there is no guest using 64 KiB guest pages that works yet).

> IMHO, it is not, and none of the proposed solutions (either change
> blkfront or Qdisk) include any change to the Xen ABI. In this case my
> preference would be to perform the change in the backend for the reasons
> detailed above.
> 
> Anyway, I'm not going to block such a change, I just think there are
> technically better ways to solve this issue.

I'm already unhappy about the complexity of the stop-gap support for 64
KiB guest pages in Linux and if there's a way to get it working by
adding a useful missing feature to qemu's disk backend then that is what
should be done.

David

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