Re: [Qemu-devel] About virtio device hotplug in Q35! 【外域邮件.谨慎查阅】

From: "Michael S. Tsirkin" <mst@redhat.com>
To: Alex Williamson <alex.williamson@redhat.com>
Cc: "Bob Chen" <a175818323@gmail.com>,
	"Marcel Apfelbaum" <marcel@redhat.com>, 陈博 <chenbo02@meituan.com>,
	qemu-devel@nongnu.org
Subject: Re: [Qemu-devel] About virtio device hotplug in Q35! 【外域邮件.谨慎查阅】
Date: Tue, 8 Aug 2017 23:07:49 +0300	[thread overview]
Message-ID: <20170808230043-mutt-send-email-mst@kernel.org> (raw)
In-Reply-To: <20170807095224.5438ef8c@w520.home>

On Mon, Aug 07, 2017 at 09:52:24AM -0600, Alex Williamson wrote:
> I wonder if it has something to do
> with the link speed/width advertised on the switch port.  I don't think
> the endpoint can actually downshift the physical link, so lspci on the
> host should probably still show the full bandwidth capability, but
> maybe the driver is somehow doing rate limiting.  PCIe gets a little
> more complicated as we go to newer versions, so it's not quite as
> simple as exposing a different bit configuration to advertise 8GT/s,
> x16. Last I tried to do link matching it was deemed too complicated
> for something I couldn't prove at the time had measurable value.  This
> might be a good way to prove that value if it makes a difference here.
> I can't think why else you'd see such a performance difference, but
> testing to see if the KVM exit rate is significantly different could
> still be an interesting verification.

It might be easiest to just dust off that patch and see whether it
helps.

-- 
MST