linux-kernel - Re: [Xen-devel] [PATCH 4/4] xen: select grant interface version

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Message-Id: <59BBEFFC020000780017B8EF@prv-mh.provo.novell.com>
Date:   Fri, 15 Sep 2017 07:21:32 -0600
From:   "Jan Beulich" <JBeulich@...e.com>
To:     "Juergen Gross" <jgross@...e.com>
Cc:     "Andrew Cooper" <andrew.cooper3@...rix.com>,
        <xen-devel@...ts.xenproject.org>, <boris.ostrovsky@...cle.com>,
        <linux-kernel@...r.kernel.org>
Subject: Re: [Xen-devel] [PATCH 4/4] xen: select grant interface
 version

>>> On 15.09.17 at 15:00, <jgross@...e.com> wrote:
> So I've found the data I've searched in the hypervisor. The maximum
> frame number to expect can be calculated from max_page, mem_hotplug
> and the maximum physical address from cpuid node 0x80000008. If
> CONFIG_BIGMEM isn't defined in Xen it is 16TB max.
> 
> The question is how to present this value to a guest. IMHO something
> like the maximum address width similar to cpuid node 0x80000008
> would be fine. It could be above width for pv guests and the max.
> memory address of the guest for HVM guests (adding a cap for those
> wouldn't be the worst idea, I guess).
> 
> What about a new subop of the xen_version hypercall?

I don't see how that would be a good fit; instead, with the CPUID
similarity you mention, why not provide the information in one of
Xen's CPUID leaves? Otoh I wonder whether returning max_page
from XENMEM_maximum_ram_page is really a good idea, if later
on that value may increase, so perhaps that op should take
mem_hotplug into account.

Jan