Am 08.08.22 um 15:26 schrieb Rob Clark:
On Mon, Aug 8, 2022 at 4:22 AM Christian König <christian.koenig@xxxxxxx> wrote:
[SNIP]
If the virtio/virtgpu UAPI was build around the idea that this is
possible then it is most likely fundamental broken.
How else can you envision mmap'ing to guest userspace working?
Well long story short: You can't.
See userspace mappings are not persistent, but rather faulted in on
demand. The exporter is responsible for setting those up to be able to
add reverse tracking and so can invalidate those mappings when the
backing store changes.
I think that is not actually a problem. At least for how it works on
arm64 but I'm almost positive x86 is similar.. I'm not sure how else
you could virtualize mmu/iommu/etc in a way that didn't have horrible
performance.
There are two levels of pagetable translation, the first controlled by
the host kernel, the second by the guest. From the PoV of host
kernel, it is just memory mapped to userspace, getting faulted in on
demand, just as normal. First the guest controlled translation
triggers a fault in the guest which sets up guest mapping. And then
the second level of translation to translate from what guest sees as
PA (but host sees as VA) to actual PA triggers a fault in the host.
Ok, that's calming.
At least that's not the approach talked about the last time this came up
and it doesn't rip a massive security hole somewhere.
The question is why is the guest then not using the caching attributes
setup by the host page tables when the translation is forwarded anyway?
[SNIP]
This is basically what happens, although via the two levels of pgtable
translation. This patch provides the missing piece, the caching
attributes.
Yeah, but that won't work like this. See the backing store migrates all
the time and when it is backed by PCIe/VRAM/local memory you need to use
write combine while system memory is usually cached.
Because otherwise you can't accommodate that the exporter is
changing those caching attributes.
Changing the attributes dynamically isn't going to work.. or at least
not easily. If you had some sort of synchronous notification to host
userspace, it could trigger an irq to the guest, I suppose. But it
would mean host kernel has to block waiting for host userspace to
interrupt the guest, then wait for guest vgpu process to be scheduled
and handle the irq.
We basically change that on every page flip on APUs and that doesn't
sound like something fast.
Thanks for the explanation how this works,
Christian.
At least in the case of msm, the cache attributes are static for the
life of the buffer, so this scenario isn't a problem. AFAICT this
should work fine for at least all UMA hw.. I'm a bit less sure when it
comes to TTM, but shouldn't you at least be able to use worst-cache
cache attributes for buffers that are allowed to be mapped to guest?
BR,
-R
But more seriously, let's take a step back here.. what scenarios are
you seeing this being problematic for? Then we can see how to come up
with solutions. The current situation of host userspace VMM just
guessing isn't great.
Well "isn't great" is a complete understatement. When KVM/virtio/virtgpu
is doing what I guess they are doing here then that is a really major
security hole.
And sticking our heads in the sand and
pretending VMs don't exist isn't great. So what can we do? I can
instead add a msm ioctl to return this info and solve the problem even
more narrowly for a single platform. But then the problem still
remains on other platforms.
Well once more: This is *not* MSM specific, you just absolutely *can't
do that* for any driver!
I'm just really wondering what the heck is going on here, because all of
this was discussed in lengthy before on the mailing list and very
bluntly rejected.
Either I'm missing something (that's certainly possible) or we have a
strong case of somebody implementing something without thinking about
all the consequences.
Regards,
Christian.
Slightly implicit in this is that mapping dma-bufs to the guest won't
work for anything that requires DMA_BUF_IOCTL_SYNC for coherency.. we
could add a possible return value for DMA_BUF_INFO_VM_PROT indicating
that the buffer does not support mapping to guest or CPU access
without DMA_BUF_IOCTL_SYNC. Then at least the VMM can fail gracefully
instead of subtly.
BR,
-R
