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

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Message-ID: <302b0b5f-34d2-07e8-ffd1-21b561daa6e2@suse.com>
Date:   Fri, 15 Sep 2017 15:27:06 +0200
From:   Juergen Gross <jgross@...e.com>
To:     Jan Beulich <JBeulich@...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 15:21, Jan Beulich wrote:
>>>> 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.

I think Andrew had concerns with the exact value being returned via
XENMEM_maximum_ram_page. Using a Xen CPUID leaf returning just the
number of address bits would be a better fit, I guess.


Juergen