[RFC PATCH 0/3] KVM: x86: honor guest memory type

[Sean Christopherson]

On Thu, Feb 20, 2020 at 09:39:05PM -0800, Tian, Kevin wrote:
> > From: Chia-I Wu <olvaffe at gmail.com>
> > Sent: Friday, February 21, 2020 12:51 PM
> > If you think it is the best for KVM to inspect hva to determine the memory
> > type with page granularity, that is reasonable and should work for us too.
> > The userspace can do something (e.g., add a GPU driver dependency to the
> > hypervisor such that the dma-buf is imported as a GPU memory and mapped
> > using
> > vkMapMemory) or I can work with dma-buf maintainers to see if dma-buf's
> > semantics can be changed.
> 
> I think you need consider the live migration requirement as Paolo pointed out.
> The migration thread needs to read/write the region, then it must use the
> same type as GPU process and guest to read/write the region. In such case, 
> the hva mapped by Qemu should have the desired type as the guest. However,
> adding GPU driver dependency to Qemu might trigger some concern. I'm not
> sure whether there is generic mechanism though, to share dmabuf fd between GPU
> process and Qemu while allowing Qemu to follow the desired type w/o using
> vkMapMemory...

Alternatively, KVM could make KVM_MEM_DMA and KVM_MEM_LOG_DIRTY_PAGES
mutually exclusive, i.e. force a transition to WB memtype for the guest
(with appropriate zapping) when migration is activated.  I think that
would work?

> Note this is orthogonal to whether introducing a new uapi or implicitly checking
> hva to favor guest memory type. It's purely about Qemu itself. Ideally anyone 
> with the desire to access a dma-buf object should follow the expected semantics.
> It's interesting that dma-buf sub-system doesn't provide a centralized 
> synchronization about memory type between multiple mmap paths.