Re: [Qemu-devel] [RFC/RFT PATCH v2 1/3] arm/arm64: pageattr: add set_memory_nc

[Christoffer Dall <christoffer.dall@linaro.org>]

On Tue, May 19, 2015 at 12:18:54PM +0100, Catalin Marinas wrote:
> On Tue, May 19, 2015 at 11:03:22AM +0100, Andrew Jones wrote:
> > On Mon, May 18, 2015 at 04:53:03PM +0100, Catalin Marinas wrote:
> > > Another way would be to split the vma containing the non-cacheable
> > > memory so that you get a single vma with the vm_page_prot as
> > > Non-cacheable.
> > 
> > This sounds interesting. Actually, it even crossed my mind once when I
> > first saw that the vma would overwrite the attributes, but then, sigh,
> > I let my brain take a stupidity bath.
> > 
> > > 
> > > Yet another approach could be for KVM to mmap the necessary memory for
> > > Qemu via a file_operations.mmap call (but that's only for ranges outside
> > > the guest "RAM").
> > 
> > I guess I prefer the vma splitting, rather than this (the vma creating
> > with mmap), as it keeps the KVM interface from changing (as you point out
> > below). Well, unless there are other advantages to this that are worth
> > considering?
> 
> The advantage is that you don't need to deal with the mm internals in
> the KVM code.
> 
> But you can probably add such code directly to mm/ and reuse some of the
> existing code in there already as part of change_protection(),
> mprotect_fixup(), sys_mprotect(). Actually, once you split the vma and
> set the new protection (something similar to mprotect_fixup), it looks
> to me like you can just call change_protection(vma->vm_page_prot).
> 
> > > I didn't have time to follow these threads in details, but just to
> > > recap my understanding, we have two main use-cases:
> > > 
> > > 1. Qemu handling guest I/O to device (e.g. PCIe BARs)
> > > 2. Qemu emulating device DMA
> > > 
> > > For (1), I guess Qemu uses an anonymous mmap() and then tells KVM about
> > > this memory slot. The memory attributes in this case could be Device
> > > because that's how the guest would normally map it. The
> > > file_operations.mmap trick would work in this case but this means
> > > expanding the KVM ABI beyond just an ioctl().
> > > 
> > > For (2), since Qemu is writing to the guest "RAM" (e.g. video
> > > framebuffer allocated by the guest), I still think the simplest is to
> > > tell the guest (via DT) that such device is cache coherent rather than
> > > trying to remap the Qemu mapping as non-cacheable.
> > 
> > If we need a solution for (1), then I'd prefer that it work and be
> > applied to (2) as well. Anyway, I'm still not 100% sure we can count on
> > all guest types (booloaders, different OSes) to listen to us. They may
> > assume non-cacheable is typical and safe, and thus just do that always.
> > We can certainly change some of those bootloaders and OSes, but probably
> > not all of them.
> 
> That's fine by me. Once you get the vma splitting and attributes
> changing done, I think you get the second one for free.
> 
> Do we want to differentiate between Device and Normal Non-cacheable
> memory? Something like KVM_MEMSLOT_DEVICE?
> 
> Nitpick: I'm not sure whether "uncached" is clear enough. In Linux,
> pgprot_noncached() returns Strongly Ordered memory. For Normal
> Non-cachable we used pgprot_writecombine (e.g. a video framebuffer).
> 
> Maybe something like KVM_MEMSLOT_COHERENT meaning a request to KVM to
> ensure that guest and host access it coherently (which would mean
> writecombine for ARM). That's similar naming to functions like
> dma_alloc_coherent() that return cacheable or non-cacheable memory based
> on what the device supports. Anyway, I'm not to bothered with the
> naming.
> 
One thing to keep in mind for (2) is that QEMU is likely to do things
like calling regular memcpy() on the memory region, so mapping it as
device memory which would fault on unaligned accesses may be a problem,
so ideally there is a memory type for the user space mapping which
allows such behavior where we at the same time can guarantee the that
the mapping is coherent with the guest mapping through the S2
attributes.

-Christoffer