Re: [PATCH 0/6] MTRR/PAT support for EPT (v3)

[Avi Kivity <avi@redhat.com>]

Sheng Yang wrote:
> Yes... But it's easy to do with assigned devices' mmio, but what if guest 
> specific some non-mmio memory's memory type? E.g. we have met one issue in 
> Xen, that a assigned-device's XP driver specific one memory region as buffer, 
> and modify the memory type then do DMA.
>
> Only map MMIO space can be first step, but I guess we can modify assigned 
> memory region memory type follow guest's? 
>   

With ept/npt, we can't, since the memory type is in the guest's 
pagetable entries, and these are not accessible.

Looks like a conflict between the requirements of a hypervisor 
supporting device assignment, and the memory type constraints of mapping 
everything with the same memory type.  As far as I can see, the only 
solution is not to map guest memory in the hypervisor, and do all 
accesses via dma.  This is easy for virtual disk, somewhat harder for 
virtual networking (need a dma engine or a multiqueue device).

Since qemu will only access memory on demand, we don't actually have to 
unmap guest memory, only to ensure that qemu doesn't touch it.  Things 
like live migration and page sharing won't work, but they aren't 
expected to with device assignment anyway.

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