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Re: [Qemu-devel] [PATCH v4 2/2] vhost-user: Attempt to fix a race with s

From: Prerna Saxena
Subject: Re: [Qemu-devel] [PATCH v4 2/2] vhost-user: Attempt to fix a race with set_mem_table.
Date: Thu, 28 Jul 2016 07:09:25 +0000

On 27/07/16 7:00 pm, "Michael S. Tsirkin" <address@hidden> wrote:

>On Wed, Jul 27, 2016 at 02:52:37AM -0700, Prerna Saxena 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>
>Could you pls reorder patchset so this is 1/2?
>1/1 is still under review but I'd like to make sure
>we have some kind of fix in place for 2.7.

Hi Michael,
The review comments for patch 1 were around documentation and the choice of 
name of flag.
There has been no recommendation/comment on the code itself.
I have fixed all of that and posted a new patch series. (Version v5.1)
Hope both the patches make it in time for 2.7.

Thanks, once again, for reviewing this.


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