Hi, We're trying to implement Vulkan with virtio-gpu, and that API exposes the difference between cached and uncached mappings to userspace (i.e, some older GPUs can't snoop the CPU's caches). We need to make sure that the guest and host caching attributes are aligned, or there's a proper API between the virtio driver and device to ensure coherence. One issue that needs to be addressed is the caching policy is variable dependent on the VM configuration and architecture. For example, on x86, it looks like a MTRR controls whether the guest caching attribute predominates[1]. On ARM, it looks like the MMU registers control whether the guest can override the host attribute, but in general it's most restrictive attribute that makes a difference[2]. Let me if that's incorrect. I'm wondering if there's some standard kernel API to query such attributes. For example, something like arch_does_guest_attribute_matter() or arch_can_guest_flush() would do the trick. Without this, we may need to introduce VIRTIO_GPU_F_* flags set by the host, but that may make the already giant QEMU command line even bigger. [1] https://lwn.net/Articles/646712/ [2] https://events.static.linuxfound.org/sites/events/files/slides/slides_10.pdf
