On Mon, 30 Mar 2020 21:59:41 -0400 Yan Zhao <yan.y.z...@intel.com> wrote:
> On Mon, Mar 30, 2020 at 10:59:23PM +0800, Alex Williamson wrote: > > On Mon, 30 Mar 2020 02:34:02 -0400 > > Yan Zhao <yan.y.z...@intel.com> wrote: > > > > > On Mon, Mar 30, 2020 at 09:35:27AM +0800, Yan Zhao wrote: > > > > On Sat, Mar 28, 2020 at 01:25:37AM +0800, Alex Williamson wrote: > > > > > On Fri, 27 Mar 2020 11:19:34 +0000 > > > > > yan.y.z...@intel.com wrote: > > > > > > > > > > > From: Yan Zhao <yan.y.z...@intel.com> > > > > > > > > > > > > currently, vfio regions without VFIO_REGION_INFO_FLAG_WRITE are only > > > > > > read-only when VFIO_REGION_INFO_FLAG_MMAP is not set. > > > > > > > > > > > > regions with flag VFIO_REGION_INFO_FLAG_READ | > > > > > > VFIO_REGION_INFO_FLAG_MMAP > > > > > > are only read-only in host page table for qemu. > > > > > > > > > > > > This patch sets corresponding ept page entries read-only for regions > > > > > > with flag VFIO_REGION_INFO_FLAG_READ | VFIO_REGION_INFO_FLAG_MMAP. > > > > > > > > > > > > accordingly, it ignores guest write when guest writes to the > > > > > > read-only > > > > > > regions are trapped. > > > > > > > > > > > > Signed-off-by: Yan Zhao <yan.y.z...@intel.com> > > > > > > Signed-off-by: Xin Zeng <xin.z...@intel.com> > > > > > > --- > > > > > > > > > > Currently we set the r/w protection on the mmap, do I understand > > > > > correctly that the change in the vfio code below results in KVM > > > > > exiting > > > > > to QEMU to handle a write to a read-only region and therefore we need > > > > > the memory.c change to drop the write? This prevents a SIGBUS or > > > > > similar? > > > > yes, correct. the change in memory.c is to prevent a SIGSEGV in host as > > > > it's mmaped to read-only. we think it's better to just drop the writes > > > > from guest rather than corrupt the qemu. > > > > > > > > > > > > > > Meanwhile vfio_region_setup() uses the same vfio_region_ops for all > > > > > regions and vfio_region_write() would still allow writes, so if the > > > > > device were using x-no-mmap=on, I think we'd still get a write to this > > > > > region and expect the vfio device to drop it. Should we prevent that > > > > > write in QEMU as well? > > > > yes, it expects vfio device to drop it right now. > > > > As the driver sets the flag without VFIO_REGION_INFO_FLAG_WRITE, it > > > > should > > > > handle it properly. > > > > both dropping in qemu and dropping in vfio device are fine to us. > > > > we wonder which one is your preference :) > > > > The kernel and device should always do the right thing, we cannot rely > > on the user to honor the mapping, but it's also a reasonable response > > from the kernel to kill the process with a SIGSEGV if the user ignores > > the protections. So I don't think it's an either/or, the kernel needs > > to do the right thing for itself and in this case QEMU should do the > > right thing for itself, which is to drop writes for regions that don't > > support it. So in general, I agree with your patch. > > > hi Alex > so is there anything I need to do? do I need to add a write dropping in > vfio_region_write() too? if yes, do I need to keep the > trace_vfio_region_write() before dropping ? Hi Yan, Yes to both feels the most consistent, right? If we want the same behavior between mmap'd and non-mmap'd regions, QEMU should drop both writes. Your change to memory_region_ram_device_write() drops the write after tracing. For both cases though it might be worthwhile to have a separate trace recorded to indicate a dropped write, probably in place of the normal trace to avoid confusion. Thanks, Alex
