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

[George Dunlap <george.dunlap@xxxxxxxxxx>]

On 11/24/2017 08:14 AM, Jan Beulich wrote:
>>>> On 23.11.17 at 18:21, <george.dunlap@xxxxxxxxxx> wrote:
>> On 11/23/2017 11:21 AM, Jan Beulich wrote:
>>>>>> On 22.11.17 at 20:20, <george.dunlap@xxxxxxxxxx> 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.

This is an area where the underlying complexity is really
(unfortunately) exposed to the user; I think hinting that there's
something complicated is OK, even if the user isn't necessarily expected
to understand it 100%.

I've changed it to the following (minus word wrapping):

---
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 memory above the 168GiB address boundary,
this limit becomes 128GiB.
---

 -George

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