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[Qemu-devel] qemu/virtio issue due to non-atomic data access
From: |
Paul Guo |
Subject: |
[Qemu-devel] qemu/virtio issue due to non-atomic data access |
Date: |
Thu, 2 May 2013 17:12:31 +0800 |
User-agent: |
Mozilla/5.0 (Windows NT 5.1; rv:17.0) Gecko/20130328 Thunderbird/17.0.5 |
Hello,
I'm developing the qemu io support for kvm on arch/tile. During virtio-net
testing I always saw the following similar message:
"Guest moved used index from 46573 to 46592"
The guest os then exits immediately. The qemu version is 0.13.0.
Here is the code that reports the error message:
static int virtqueue_num_heads(VirtQueue *vq, unsigned int idx)
{
uint16_t num_heads = vring_avail_idx(vq) - idx;
/* Check it isn't doing very strange things with descriptor numbers. */
if (num_heads > vq->vring.num) {
fprintf(stderr, "Guest moved used index from %u to %u",
idx, vring_avail_idx(vq));
exit(1);
}
return num_heads;
}
I looked into this issue a bit, it seems that this is due to the non-atomic
data access of some virtio variables in qemu. In the above case,
vq->vring.avail.idx is modified by kernel and is read in qemu via lduw_le_p()
(for our default hw configuration case). lduw_le_p() loads the 16bit values
byte by byte. If the kernel is updating the value from 0xB5FF to 0xB600 (i.e.
46592), qemu probably reads 0xB6FF and then virtqueue_num_heads() enters the
error handling branch.
static inline int lduw_le_p(const void *ptr)
{
#ifdef _ARCH_PPC
int val;
__asm__ __volatile__ ("lhbrx %0,0,%1" : "=r" (val) : "r" (ptr));
return val;
#else
const uint8_t *p = ptr;
return p[0] | (p[1] << 8);
#endif
}
Latest qemu changes to use memcpy() in lduw_le_p(), but if the alignment of the
destination pointer in memcpy() is not implied, the compiler will probably
still have to load byte by byte, thus vring_avail_idx() still has this issue.
A proper fix for this issue seems to be: Judge whether the address is aligned,
do direct loading for the aligned case in ldq_le_p(), etc?
Thanks,
Paul
- [Qemu-devel] qemu/virtio issue due to non-atomic data access,
Paul Guo <=