Re: [Xen-devel] [PATCH] Don't track all memory when enabling log dirty to track vram

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 10/02/14 08:03, Tim Deegan wrote:
> At 14:14 +0800 on 10 Feb (1392038040), Yang Zhang wrote:
>> From: Yang Zhang <yang.z.zhang@xxxxxxxxx>
>>
>> When enabling log dirty mode, it sets all guest's memory to readonly.
>> And in HAP enabled domain, it modifies all EPT entries to clear write bit
>> to make sure it is readonly. This will cause problem if VT-d shares page
>> table with EPT: the device may issue a DMA write request, then VT-d engine
>> tells it the target memory is readonly and result in VT-d fault.
> So that's a problem even if only the VGA framebuffer is being tracked
> -- DMA from a passthrough device will either cause a spurious error or
> fail to update the dirt bitmap. 
>
> I think it would be better not to allow VT-d and EPT to share
> pagetables in cases where devices are passed through (i.e. all cases
> where VT-d is in use).
>
> Tim.

Sadly, this would make shared VT-d/EPT completely pointless as a
feature, causing extra memory overhead in Xen by having to maintain EPT
and IOMMU tables separately.

Any usecase which doesn't involve dirty vram tracking (e.g. headless VM
with SRIOV, PVH dom0) would be adversely affected.

~Andrew

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