From mboxrd@z Thu Jan 1 00:00:00 1970 From: Liu Yuan Subject: Re: [PATCH] Simulates the MSIx table read operation Date: Wed, 11 Aug 2010 00:04:15 +0800 Message-ID: Mime-Version: 1.0 Content-Type: multipart/mixed; boundary="===============1575381280==" Return-path: List-Unsubscribe: , List-Post: List-Help: List-Subscribe: , Sender: xen-devel-bounces@lists.xensource.com Errors-To: xen-devel-bounces@lists.xensource.com To: konrad.wilk@oracle.com Cc: xen-devel@lists.xensource.com List-Id: xen-devel@lists.xenproject.org --===============1575381280== Content-Type: multipart/alternative; boundary=0016e6471848e4bda9048d7a48a5 --0016e6471848e4bda9048d7a48a5 Content-Type: text/plain; charset=UTF-8 >I am having a hard time understanding this. Is the issue here that >read/write of the MSI-X table is being done in QEMU, and it is much >better to do so in the hypervisor which traps already the mask/unmaks >operation so that QEMU is not overwhelmed by having to do this? Yes, having QEMU do this read operation,it needs much more CPU cycles and longer latency. [before patch] read path: Guest -> Xen -> QEMU -> XEN -> Guest [after patch] read path: Guest -> Xen -> Guest Xen traps all the MSIx table operation. And before this patch, only mask/unmask is simulated in Xen. With this patch applied, READ will be simulated and WRITE will *still* be forwarded to QEMU. > >With this in patch in place, wouldn't QEMU still do the read operation? No Thanks, Yuan -- I am Multician, really --0016e6471848e4bda9048d7a48a5 Content-Type: text/html; charset=UTF-8 Content-Transfer-Encoding: quoted-printable 
>I am having a hard time understanding this.  Is the issue here tha=
t
>read/write of the MSI-X table is being done in QEMU, and it is muc=
h
>better to do so in the hypervisor which traps already the mask/unm=
aks

>operation so that QEMU is not overwhelmed by having to do this?

=
Yes, having QEMU do this read operation,it needs much more CPU cycles and l=
onger latency.

[before patch]
read path: Guest -> Xen -> QE=
MU -> XEN -> Guest

[after patch]
read path: Guest -> Xen -> Guest

Xen traps al=
l the MSIx table operation. And before this patch, only mask/unmask is simu=
lated in
Xen. With this patch applied, READ will be simulated and WRITE =
will *still* be forwarded to QEMU.


>
>With this in patch in place, wouldn't QEMU still do the=
 read operation?
No

Thanks,
Yuan

--=
I am Multician, really
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