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Re: [Qemu-devel] [PATCH v3 3/4] vfio: Inhibit ballooning based on group

From: Peter Xu
Subject: Re: [Qemu-devel] [PATCH v3 3/4] vfio: Inhibit ballooning based on group attachment to a container
Date: Wed, 8 Aug 2018 11:38:28 +0800
User-agent: Mutt/1.10.0 (2018-05-17)

On Tue, Aug 07, 2018 at 01:31:24PM -0600, Alex Williamson wrote:
> We use a VFIOContainer to associate an AddressSpace to one or more
> VFIOGroups.  The VFIOContainer represents the DMA context for that
> AdressSpace for those VFIOGroups and is synchronized to changes in
> that AddressSpace via a MemoryListener.  For IOMMU backed devices,
> maintaining the DMA context for a VFIOGroup generally involves
> pinning a host virtual address in order to create a stable host
> physical address and then mapping a translation from the associated
> guest physical address to that host physical address into the IOMMU.
> While the above maintains the VFIOContainer synchronized to the QEMU
> memory API of the VM, memory ballooning occurs outside of that API.
> Inflating the memory balloon (ie. cooperatively capturing pages from
> the guest for use by the host) simply uses MADV_DONTNEED to "zap"
> pages from QEMU's host virtual address space.  The page pinning and
> IOMMU mapping above remains in place, negating the host's ability to
> reuse the page, but the host virtual to host physical mapping of the
> page is invalidated outside of QEMU's memory API.
> When the balloon is later deflated, attempting to cooperatively
> return pages to the guest, the page is simply freed by the guest
> balloon driver, allowing it to be used in the guest and incurring a
> page fault when that occurs.  The page fault maps a new host physical
> page backing the existing host virtual address, meanwhile the
> VFIOContainer still maintains the translation to the original host
> physical address.  At this point the guest vCPU and any assigned
> devices will map different host physical addresses to the same guest
> physical address.  Badness.
> The IOMMU typically does not have page level granularity with which
> it can track this mapping without also incurring inefficiencies in
> using page size mappings throughout.  MMU notifiers in the host
> kernel also provide indicators for invalidating the mapping on
> balloon inflation, not for updating the mapping when the balloon is
> deflated.  For these reasons we assume a default behavior that the
> mapping of each VFIOGroup into the VFIOContainer is incompatible
> with memory ballooning and increment the balloon inhibitor to match
> the attached VFIOGroups.
> Signed-off-by: Alex Williamson <address@hidden>

Reviewed-by: Peter Xu <address@hidden>


Peter Xu

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