On Tue, Feb 22, 2022 at 12:17 AM David Woodhouse <dwmw2@xxxxxxxxxxxxx> wrote: > > As things stand, an application which wants to use vhost with a trivial > 1:1 mapping of its virtual address space is forced to jump through hoops > to detect what the address range might be. The VHOST_SET_MEM_TABLE ioctl > helpfully doesn't fail immediately; you only get a failure *later* when > you attempt to set the backend, if the table *could* map to an address > which is out of range, even if no out-of-range address is actually > being referenced. > > Since userspace is growing workarounds for this lovely kernel API, let's > ensure that we have a regression test that does things basically the same > way as https://gitlab.com/openconnect/openconnect/-/commit/443edd9d8826 > does. I wonder if it's useful to have a small library that wraps vhost kernel uAPI somewhere. (In the future, we may want to let the kernel accept 1:1 mapping by figuring out the illegal range by itself?) Thanks > > This is untested as I can't actually get virtio_test to work at all; it > just seems to deadlock on a spinlock. But it's getting the right answer > for the virtio range on x86_64 at least. > > Signed-off-by: David Woodhouse <dwmw2@xxxxxxxxxxxxx> > --- > > Please, tell me I don't need to do this. But if I *do*, it needs a > regression test in-kernel. > > tools/virtio/virtio_test.c | 109 ++++++++++++++++++++++++++++++++++++- > 1 file changed, 106 insertions(+), 3 deletions(-) > > diff --git a/tools/virtio/virtio_test.c b/tools/virtio/virtio_test.c > index cb3f29c09aff..e40eeeb05b71 100644 > --- a/tools/virtio/virtio_test.c > +++ b/tools/virtio/virtio_test.c > @@ -11,6 +11,7 @@ > #include <sys/ioctl.h> > #include <sys/stat.h> > #include <sys/types.h> > +#include <sys/mman.h> > #include <fcntl.h> > #include <stdbool.h> > #include <linux/virtio_types.h> > @@ -124,6 +125,109 @@ static void vq_info_add(struct vdev_info *dev, int num) > dev->nvqs++; > } > > +/* > + * This is awful. The kernel doesn't let us just ask for a 1:1 mapping of > + * our virtual address space; we have to *know* the minimum and maximum > + * addresses. We can't test it directly with VHOST_SET_MEM_TABLE because > + * that actually succeeds, and the failure only occurs later when we try > + * to use a buffer at an address that *is* valid, but our memory table > + * *could* point to addresses that aren't. Ewww. > + * > + * So... attempt to work out what TASK_SIZE is for the kernel we happen > + * to be running on right now... > + */ > + > +static int testaddr(unsigned long addr) > +{ > + void *res = mmap((void *)addr, getpagesize(), PROT_NONE, > + MAP_FIXED|MAP_ANONYMOUS, -1, 0); > + if (res == MAP_FAILED) { > + if (errno == EEXIST || errno == EINVAL) > + return 1; > + > + /* We get ENOMEM for a bad virtual address */ > + return 0; > + } > + /* It shouldn't actually succeed without either MAP_SHARED or > + * MAP_PRIVATE in the flags, but just in case... */ > + munmap((void *)addr, getpagesize()); > + return 1; > +} > + > +static int find_vmem_range(struct vhost_memory *vmem) > +{ > + const unsigned long page_size = getpagesize(); > + unsigned long top; > + unsigned long bottom; > + > + top = -page_size; > + > + if (testaddr(top)) { > + vmem->regions[0].memory_size = top; > + goto out; > + } > + > + /* 'top' is the lowest address known *not* to work */ > + bottom = top; > + while (1) { > + bottom >>= 1; > + bottom &= ~(page_size - 1); > + assert(bottom); > + > + if (testaddr(bottom)) > + break; > + top = bottom; > + } > + > + /* It's often a page or two below the boundary */ > + top -= page_size; > + if (testaddr(top)) { > + vmem->regions[0].memory_size = top; > + goto out; > + } > + top -= page_size; > + if (testaddr(top)) { > + vmem->regions[0].memory_size = top; > + goto out; > + } > + > + /* Now, bottom is the highest address known to work, > + and we must search between it and 'top' which is > + the lowest address known not to. */ > + while (bottom + page_size != top) { > + unsigned long test = bottom + (top - bottom) / 2; > + test &= ~(page_size - 1); > + > + if (testaddr(test)) { > + bottom = test; > + continue; > + } > + test -= page_size; > + if (testaddr(test)) { > + vmem->regions[0].memory_size = test; > + goto out; > + } > + > + test -= page_size; > + if (testaddr(test)) { > + vmem->regions[0].memory_size = test; > + goto out; > + } > + top = test; > + } > + vmem->regions[0].memory_size = bottom; > + > + out: > + vmem->regions[0].guest_phys_addr = page_size; > + vmem->regions[0].userspace_addr = page_size; > + printf("Detected virtual address range 0x%lx-0x%lx\n", > + page_size, > + (unsigned long)(page_size + vmem->regions[0].memory_size)); > + > + return 0; > +} > + > + > static void vdev_info_init(struct vdev_info* dev, unsigned long long features) > { > int r; > @@ -143,9 +247,8 @@ static void vdev_info_init(struct vdev_info* dev, unsigned long long features) > memset(dev->mem, 0, offsetof(struct vhost_memory, regions) + > sizeof dev->mem->regions[0]); > dev->mem->nregions = 1; > - dev->mem->regions[0].guest_phys_addr = (long)dev->buf; > - dev->mem->regions[0].userspace_addr = (long)dev->buf; > - dev->mem->regions[0].memory_size = dev->buf_size; > + r = find_vmem_range(dev->mem); > + assert(r >= 0); > r = ioctl(dev->control, VHOST_SET_MEM_TABLE, dev->mem); > assert(r >= 0); > } > > _______________________________________________ Virtualization mailing list Virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linuxfoundation.org/mailman/listinfo/virtualization
