Re: [Qemu-devel] [PATCHv2 1/3] virtio: add missing mb() on notification

[Paolo Bonzini]

Il 23/04/2012 15:19, Michael S. Tsirkin ha scritto:
> During normal operation, virtio first writes a used index
> and then checks whether it should interrupt the guest
> by reading guest avail index/flag values.
> 
> Guest does the reverse: writes the index/flag,
> then checks the used ring.
> 
> The ordering is important: if host avail flag read bypasses the used
> index write, we could in effect get this timing:
> 
> host avail flag read
>               guest enable interrupts: avail flag write
>               guest check used ring: ring is empty
> host used index write
> 
> which results in a lost interrupt: guest will never be notified
> about the used ring update.
> 
> This actually can happen when using kvm with an io thread,
> such that the guest vcpu and qemu run on different host cpus,
> and this has actually been observed in the field
> (but only seems to trigger on very specific processor types)
> with userspace virtio: vhost has the necessary smp_mb()
> in place to prevent the regordering, so the same workload stalls
> forever waiting for an interrupt with vhost=off but works
> fine with vhost=on.
> 
> Insert an smp_mb barrier operation in userspace virtio to
> ensure the correct ordering.
> Applying this patch fixed the race condition we have observed.
> Tested on x86_64. I checked the code generated by the new macro
> for i386 and ppc but didn't run virtio.
> 
> Signed-off-by: Michael S. Tsirkin <address@hidden>
> ---
>  hw/virtio.c    |    2 ++
>  qemu-barrier.h |   23 ++++++++++++++++++++---
>  2 files changed, 22 insertions(+), 3 deletions(-)
> 
> diff --git a/hw/virtio.c b/hw/virtio.c
> index f805790..6449746 100644
> --- a/hw/virtio.c
> +++ b/hw/virtio.c
> @@ -693,6 +693,8 @@ static bool vring_notify(VirtIODevice *vdev, VirtQueue 
> *vq)
>  {
>      uint16_t old, new;
>      bool v;
> +    /* We need to expose used array entries before checking used event. */
> +    mb();

mb() vs. smp_mb()?

>      /* Always notify when queue is empty (when feature acknowledge) */
>      if (((vdev->guest_features & (1 << VIRTIO_F_NOTIFY_ON_EMPTY)) &&
>           !vq->inuse && vring_avail_idx(vq) == vq->last_avail_idx)) {
> diff --git a/qemu-barrier.h b/qemu-barrier.h
> index c11bb2b..f6722a8 100644
> --- a/qemu-barrier.h
> +++ b/qemu-barrier.h
> @@ -4,7 +4,7 @@
>  /* Compiler barrier */
>  #define barrier()   asm volatile("" ::: "memory")
>  
> -#if defined(__i386__) || defined(__x86_64__)
> +#if defined(__i386__)
>  
>  /*
>   * Because of the strongly ordered x86 storage model, wmb() is a nop
> @@ -13,15 +13,31 @@
>   * load/stores from C code.
>   */
>  #define smp_wmb()   barrier()
> +/*
> + * We use GCC builtin if it's available, as that can use
> + * mfence on 32 bit as well, e.g. if built with -march=pentium-m.
> + * However, on i386, there seem to be known bugs as recently as 4.3.
> + * */

Do you know what those bugs are?  Either add a pointer, or there is no
reason to have cruft that is only backed by hearsay.

> +#if defined(_GNUC__) && __GNUC__ >= 4 && __GNUC_MINOR__ >= 4
> +#define smp_mb() __sync_synchronize()
> +#else
> +#define smp_mb() asm volatile("lock; addl $0,0(%%esp) " ::: "memory")
> +#endif
> +
> +#elif defined(__x86_64__)
> +
> +#define smp_wmb()   barrier()
> +#define smp_mb() asm volatile("mfence" ::: "memory")
>  
>  #elif defined(_ARCH_PPC)
>  
>  /*
> - * We use an eieio() for a wmb() on powerpc.  This assumes we don't
> + * We use an eieio() for wmb() and mb() on powerpc.  This assumes we don't
>   * need to order cacheable and non-cacheable stores with respect to
>   * each other
>   */
>  #define smp_wmb()   asm volatile("eieio" ::: "memory")
> +#define smp_mb()   asm volatile("eieio" ::: "memory")

smp_mb() is hwsync under PPC, but I would just trust GCC.

Paolo

>  
>  #else
>  
> @@ -29,9 +45,10 @@
>   * For (host) platforms we don't have explicit barrier definitions
>   * for, we use the gcc __sync_synchronize() primitive to generate a
>   * full barrier.  This should be safe on all platforms, though it may
> - * be overkill.
> + * be overkill for wmb().
>   */
>  #define smp_wmb()   __sync_synchronize()
> +#define smp_mb()   __sync_synchronize()
>  
>  #endif
>