Re: [PATCH v3 16/17] SUPPORT.md: Add limits RFC

From: "Jan Beulich" <JBeulich@suse.com>
To: George Dunlap <george.dunlap@citrix.com>
Cc: StefanoStabellini <sstabellini@kernel.org>,
	Wei Liu <wei.liu2@citrix.com>,
	Konrad Wilk <konrad.wilk@oracle.com>,
	Andrew Cooper <andrew.cooper3@citrix.com>,
	Tim Deegan <tim@xen.org>, Ian Jackson <ian.jackson@citrix.com>,
	xen-devel@lists.xenproject.org
Subject: Re: [PATCH v3 16/17] SUPPORT.md: Add limits RFC
Date: Fri, 24 Nov 2017 01:14:31 -0700	[thread overview]
Message-ID: <5A17E2F70200007800191937@prv-mh.provo.novell.com> (raw)
In-Reply-To: <72015cfa-3a5b-99e0-ef58-4782151eaa77@citrix.com>

>>> On 23.11.17 at 18:21, <george.dunlap@citrix.com> wrote:
> On 11/23/2017 11:21 AM, Jan Beulich wrote:
>>>>> On 22.11.17 at 20:20, <george.dunlap@citrix.com> wrote:
>>> +### Virtual RAM
>>> +
>>> +    Limit-security, x86 PV 64-bit: 2047GiB
>>> +    Limit-security, x86 PV 32-bit: 168GiB (see below)
>>> +    Limit-security, x86 HVM: 1.5TiB
>>> +    Limit, ARM32: 16GiB
>>> +    Limit, ARM64: 1TiB
>>> +
>>> +Note that there are no theoretical limits to 64-bit PV or HVM guest sizes
>>> +other than those determined by the processor architecture.
>>> +
>>> +All 32-bit PV guest memory must be under 168GiB;
>>> +this means the total memory for all 32-bit PV guests cannot exced 168GiB.
>> 
>> While certainly harder to grok for the reader, I think we need to be
>> precise here: The factor isn't the amount of memory, but the
>> addresses at which it surfaces. Host memory must not extend
>> beyond the 168MiB boundary for that to also be the limit for
>> 32-bit PV guests.
> 
> Yes, I'd intended "under 168GiB" to more clearly imply physical
> addresses; but I agree as written that's unlikely to be picked up by
> anyone not already familiar with the concept.
> 
> What about something like this:
> 
> "32-bit PV guests can only access physical addresses below 168GiB;
> this means that the total memory of all 32-bit PV guests cannot exceed
> 168GiB.  For hosts with more than 168GiB RAM, this limit becomes 128GiB."

Better, but I'd still prefer the last sentence to be something like
"For hosts with memory above the 168GiB boundary, ...". I'm
not going to insist on such an adjustment, though, if you feel
what you have is easier to understand by the intended audience.

Jan

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