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Re: [Qemu-devel] [PULL 3/3] vhost-user: Attempt to fix a race with set_m
From: |
Michael S. Tsirkin |
Subject: |
Re: [Qemu-devel] [PULL 3/3] vhost-user: Attempt to fix a race with set_mem_table. |
Date: |
Sun, 14 Aug 2016 05:51:06 +0300 |
On Fri, Aug 12, 2016 at 07:16:34AM +0000, Prerna Saxena wrote:
>
> On 12/08/16 12:08 pm, "Fam Zheng" <address@hidden> wrote:
>
>
>
>
>
> >On Wed, 08/10 18:30, Michael S. Tsirkin wrote:
> >> From: Prerna Saxena <address@hidden>
> >>
> >> The set_mem_table command currently does not seek a reply. Hence, there is
> >> no easy way for a remote application to notify to QEMU when it finished
> >> setting up memory, or if there were errors doing so.
> >>
> >> As an example:
> >> (1) Qemu sends a SET_MEM_TABLE to the backend (eg, a vhost-user net
> >> application). SET_MEM_TABLE does not require a reply according to the spec.
> >> (2) Qemu commits the memory to the guest.
> >> (3) Guest issues an I/O operation over a new memory region which was
> >> configured on (1).
> >> (4) The application has not yet remapped the memory, but it sees the I/O
> >> request.
> >> (5) The application cannot satisfy the request because it does not know
> >> about those GPAs.
> >>
> >> While a guaranteed fix would require a protocol extension (committed
> >> separately),
> >> a best-effort workaround for existing applications is to send a
> >> GET_FEATURES
> >> message before completing the vhost_user_set_mem_table() call.
> >> Since GET_FEATURES requires a reply, an application that processes
> >> vhost-user
> >> messages synchronously would probably have completed the SET_MEM_TABLE
> >> before replying.
> >>
> >> Signed-off-by: Prerna Saxena <address@hidden>
> >> Reviewed-by: Michael S. Tsirkin <address@hidden>
> >> Signed-off-by: Michael S. Tsirkin <address@hidden>
> >
> >Sporadic hangs are seen with test-vhost-user after this patch:
> >
> >https://travis-ci.org/qemu/qemu/builds
> >
> >Reverting seems to fix it for me.
> >
> >Is this a known problem?
> >
> >Fam
>
> Hi Fam,
> Thanks for reporting the sporadic hangs. I had seen ‘make check’ pass on my
> Centos 6 environment, so missed this.
> I am setting up the docker test env to repro this, but I think I can guess
> the problem :
>
> In tests/vhost-user-test.c:
>
> static void chr_read(void *opaque, const uint8_t *buf, int size)
> {
> ..[snip]..
>
> case VHOST_USER_SET_MEM_TABLE:
> /* received the mem table */
> memcpy(&s->memory, &msg.payload.memory, sizeof(msg.payload.memory));
> s->fds_num = qemu_chr_fe_get_msgfds(chr, s->fds, G_N_ELEMENTS(s->fds));
>
>
> /* signal the test that it can continue */
> g_cond_signal(&s->data_cond);
> break;
> ..[snip]..
> }
>
>
> The test seems to be marked complete as soon as mem_table is copied.
> However, this patch 3/3 changes the behaviour of the SET_MEM_TABLE vhost
> command implementation with qemu. SET_MEM_TABLE now sends out a new message
> GET_FEATURES, and the call is only completed once it receives features from
> the remote application. (or the test framework, as is the case here.)
Hmm but why does it matter that data_cond is woken up?
> While the test itself can be modified (Do not signal completion until we’ve
> sent a follow-up response to GET_FEATURES), I am now wondering if this patch
> may break existing vhost applications too ? If so, reverting it possibly
> better.
What bothers me is that the new feature might cause the same
issue once we enable it in the test.
How about a patch to tests/vhost-user-test.c adding the new
protocol feature? I would be quite interested to see what
is going on with it.
> What confuses me is why it doesn’t fail all the time, but only about 20% to
> 30% time as Fam reports.
And succeeds every time on my systems :(
>
> Thoughts : Michael, Fam, MarcAndre ?
>
> Regards,
> Prerna
- Re: [Qemu-devel] [PULL 3/3] vhost-user: Attempt to fix a race with set_mem_table., (continued)
- Re: [Qemu-devel] [PULL 3/3] vhost-user: Attempt to fix a race with set_mem_table., Michael S. Tsirkin, 2016/08/12
- Re: [Qemu-devel] [PULL 3/3] vhost-user: Attempt to fix a race with set_mem_table., Marc-André Lureau, 2016/08/12
- Re: [Qemu-devel] [PULL 3/3] vhost-user: Attempt to fix a race with set_mem_table., Michael S. Tsirkin, 2016/08/12
- Re: [Qemu-devel] [PULL 3/3] vhost-user: Attempt to fix a race with set_mem_table., Marc-André Lureau, 2016/08/13
- Re: [Qemu-devel] [PULL 3/3] vhost-user: Attempt to fix a race with set_mem_table., Michael S. Tsirkin, 2016/08/13
- Re: [Qemu-devel] [PULL 3/3] vhost-user: Attempt to fix a race with set_mem_table., Michael S. Tsirkin, 2016/08/13
- Re: [Qemu-devel] [PULL 3/3] vhost-user: Attempt to fix a race with set_mem_table.,
Michael S. Tsirkin <=
- Re: [Qemu-devel] [PULL 3/3] vhost-user: Attempt to fix a race with set_mem_table., Prerna Saxena, 2016/08/14
- Re: [Qemu-devel] [PULL 3/3] vhost-user: Attempt to fix a race with set_mem_table., Michael S. Tsirkin, 2016/08/14
- Re: [Qemu-devel] [PULL 3/3] vhost-user: Attempt to fix a race with set_mem_table., Maxime Coquelin, 2016/08/31
Re: [Qemu-devel] [PULL 0/3] virtio/vhost: fixes, Peter Maydell, 2016/08/10