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

[Tim Deegan <tim@xxxxxxx>]

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.

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