[RFC/RFT PATCH 0/3] arm64: KVM: work around incoherency with uncached guest mappings

[Mario Smarduch]

On 03/05/2015 09:43 AM, Paolo Bonzini wrote:
> 
> 
> On 05/03/2015 15:58, Catalin Marinas wrote:
>>> It would especially suck if the user has a cluster with different
>>> machines, some of them coherent and others non-coherent, and then has to
>>> debug why the same configuration works on some machines and not on others.
>>
>> That's a problem indeed, especially with guest migration. But I don't
>> think we have any sane solution here for the bus master DMA.
> 
> I do not oppose doing cache management in QEMU for bus master DMA
> (though if the solution you outlined below works it would be great).
> 
>> ARM can override them as well but only making them stricter. Otherwise,
>> on a weakly ordered architecture, it's not always safe (let's say the
>> guest thinks it accesses Strongly Ordered memory and avoids barriers for
>> flag updates but the host "upgrades" it to Cacheable which breaks the
>> memory order).
> 
> The same can happen on x86 though, even if it's rarer.  You still need a
> barrier between stores and loads.
> 
>> If we want the host to enforce guest memory mapping attributes via stage
>> 2, we could do it the other way around: get the guests to always assume
>> full cache coherency, generating Normal Cacheable mappings, but use the
>> stage 2 attributes restriction in the host to make such mappings
>> non-cacheable when needed (it works this way on ARM but not in the other
>> direction to relax the attributes).
> 
> That sounds like a plan for device assignment.  But it still would not
> solve the problem of the MMIO framebuffer, right?
> 
>>> The problem arises with MMIO areas that the guest can reasonably expect
>>> to be uncacheable, but that are optimized by the host so that they end
>>> up backed by cacheable RAM.  It's perfectly reasonable that the same
>>> device needs cacheable mapping with one userspace, and works with
>>> uncacheable mapping with another userspace that doesn't optimize the
>>> MMIO area to RAM.
>>
>> Unless the guest allocates the framebuffer itself (e.g.
>> dma_alloc_coherent), we can't control the cacheability via
>> "dma-coherent" properties as it refers to bus master DMA.
> 
> Okay, it's good to rule that out.  One less thing to think about. :)
> Same for _DSD.
> 
>> So for MMIO with the buffer allocated by the host (Qemu), the only
>> solution I see on ARM is for the host to ensure coherency, either via
>> explicit cache maintenance (new KVM API) or by changing the memory
>> attributes used by Qemu to access such virtual MMIO.
>>
>> Basically Qemu is acting as a bus master when reading the framebuffer it
>> allocated but the guest considers it a slave access and we don't have a
>> way to tell the guest that such accesses should be cacheable, nor can we
>> upgrade them via architecture features.
> 
> Yes, that's a way to put it.
> 
>>> In practice, the VGA framebuffer has an optimization that uses dirty
>>> page tracking, so we could piggyback on the ioctls that return which
>>> pages are dirty.  It turns out that piggybacking on those ioctls also
>>> should fix the case of migrating a guest while the MMU is disabled.
>>
>> Yes, Qemu would need to invalidate the cache before reading a dirty
>> framebuffer page.
>>
>> As I said above, an API that allows non-cacheable mappings for the VGA
>> framebuffer in Qemu would also solve the problem. I'm not sure what KVM
>> provides here (or whether we can add such API).
> 
> Nothing for now; other architectures simply do not have the issue.
> 
> As long as it's just VGA, we can quirk it.  There's just a couple
> vendor/device IDs to catch, and the guest can then use a cacheable mapping.
> 
> For a more generic solution, the API would be madvise(MADV_DONTCACHE).
> It would be easy for QEMU to use it, but I am not too optimistic about
> convincing the mm folks about it.  We can try.

Interested to see the outcome.

I was thinking of a very basic memory driver that can provide
an uncached memslot to QEMU - in mmap() file operation
apply pgprot_uncached to allocated pages, lock them, flush TLB
call remap_pfn_range().

Mario

> 
> Paolo
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