Re: [PATCH 0/4] CPU hotplug port from qemu-traditionnal to qemu-xen for 4.3.

From: Anthony PERARD <anthony.perard@citrix.com>
To: Ian Campbell <Ian.Campbell@citrix.com>
Cc: Stefano Stabellini <stefano.stabellini@citrix.com>,
	Xen Devel <xen-devel@lists.xen.org>
Subject: Re: [PATCH 0/4] CPU hotplug port from qemu-traditionnal to qemu-xen for 4.3.
Date: Mon, 3 Jun 2013 12:12:52 +0100	[thread overview]
Message-ID: <51AC7A34.3010102@citrix.com> (raw)
In-Reply-To: <1370248908.404.11.camel@zakaz.uk.xensource.com>

On 03/06/13 09:41, Ian Campbell wrote:
>> > BUG: There is an issue with SeaBIOS (used with qemu-xen). At SMP
>> > initialisation, SeaBIOS will count the number of CPU running, and the number is
>> > always equal to maxvcpu when SeaBIOS expect less. This happen when we have
>> > something like:
>> > vcpus = 2
>> > maxvcpus = 8
>> > in the VM config file. Linux is fine with this and will use only $vcpus.  So
>> > the probleme is: an infinit loop in SeaBIOS.
> I'm a bit confused -- if SeaBIOS has an infinite loop how do you even
> get to know Linux is OK?
> 
> Do you have a fix for SeaBIOS, I don't see it in this series.

The thing is, I don't know how to fix this bug.  This is why there is no
patch.

The probleme is this loop:
> u8 cmos_smp_count = inb_cmos(CMOS_BIOS_SMP_COUNT);
> while (cmos_smp_count + 1 != readl(&CountCPUs))
>   ;

where CountCPUs is incremented by each CPU by a small piece of code and
cmos_smp_count is equal to 1, which is the number of expected vcpus-1.
CountCPUs always go up to maxvcpus under Xen.

I could simply replace the '!=' by a '>' but it feel like a workaround...

-- 
Anthony PERARD