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Re: [Qemu-devel] [PATCH] vhost-user spec: Clarify policy on setting log_

From: Michael S. Tsirkin
Subject: Re: [Qemu-devel] [PATCH] vhost-user spec: Clarify policy on setting log_base
Date: Tue, 28 Nov 2017 20:06:52 +0200

On Tue, Nov 28, 2017 at 03:46:44PM +0200, Victor Kaplansky wrote:
> From: Victor Kaplansky <address@hidden>
> If we allow qemu to change logging area after it was already established,
> it may require from the backend to acquire a lock on each access to
> the log_base, which has a potential quite a big performance hit.
> Thus we would like to clarify in the spec, that qemu is not expected
> to resize or remap the logging area, and backend implementations
> can safely ignore subsequent requests to log_base modifications.
> Signed-off-by: Victor Kaplansky <address@hidden>
> Suggested-by: Maxime Coquelin <address@hidden>

I'm not sure we can do this.

Log resizing as a result of memory hotplug might force
log base changes.

Backends need to use something like
rcu to avoid need for locking.

Apropos I wonder whether it's a bug that vhost_dev_start
calls vhost_set_log_base after starting rings.

Same question for the iotlb callback.

> ---
>  docs/interop/vhost-user.txt | 6 ++++++
>  1 file changed, 6 insertions(+)
> diff --git a/docs/interop/vhost-user.txt b/docs/interop/vhost-user.txt
> index 954771d0d8..7ab31e57ef 100644
> --- a/docs/interop/vhost-user.txt
> +++ b/docs/interop/vhost-user.txt
> @@ -257,6 +257,12 @@ Where addr is the guest physical address.
>  Use atomic operations, as the log may be concurrently manipulated.
> +Note that master is not expected to issue more than one 
> +request before the rings are fully stopped by the master. Thus no 
> modifications
> +to log_base address are allowed before the rings are restated and the client
> +can ignore all subsequent VHOST_USER_SET_LOG_BASE requests after the log_base
> +address has been established.
> +
>  Note that when logging modifications to the used ring (when VHOST_VRING_F_LOG
>  is set for this ring), log_guest_addr should be used to calculate the log
>  offset: the write to first byte of the used ring is logged at this offset 
> from
> -- 
> 2.14.2

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