Re: [Xen-devel] [PATCH v2 10/10] xen/arm: add reserved-memory regions to the dom0 memory node

[Julien Grall <Julien.Grall@arm.com>]

Hi Stefano,

On 20/05/2019 22:26, Stefano Stabellini wrote:
> On Sat, 11 May 2019, Julien Grall wrote:
>>>>> But I am still not happy with the approach taken for the reserved-memory
>>>>> regions in this series. As I pointed out before, they are just normal
>>>>> memory
>>>>> that was reserved for other purpose (CMA, framebuffer...).
>>>>>
>>>>> Treating them as "device" from Xen POV is a clear abuse of the meaning
>>>>> and I
>>>>> don't believe it is a viable solution long term.
>>>>
>>>> If we don't consider "reusable" memory regions as part of the
>>>> discussion, the distinction becomes more philosophical than practical:
>>>>
>>>> - Xen is not supposed to use them for anything
>>>> - only given them to the VM configured for it
>>>>
>>>> I don't see much of a difference with MMIO regions, except for the
>>>> expected pagetable attributes: i.e. cacheable, not-cacheable. But even
>>>> in that case, there could be reasonable use cases for non-cacheable
>>>> mappings of reserved-memory regions, even if reserved-memory regions are
>>>> "normal" memory.
>>>>
>>>> Could you please help me understand why you see them so differently, as
>>>> far as to say that "treating them as "device" from Xen POV is a clear
>>>> abuse of the meaning"?
>>>
>>> Obviously if you take half of the picture, then it makes things easier.
>>> However, we are not here to discuss half of the picture but the full one
>>> (even if at the end you only implement half of it).
>>>
>>>>> Indeed, some of the regions may have a property "reusable" allowing the
>>>>> the OS
>>>>> to use them until they are claimed by the device driver owning the
>>>>> region. I
>>>>> don't know how Linux (or any other OS) is using it today, but I don't
>>>>> see what
>>>>> would prevent it to use them as hypercall buffer. This would obviously
>>>>> not
>>>>> work because they are not actual RAM from Xen POV.
>>>>
>>>> I haven't attempted at handling "reusable" reserved-memory regions
>>>> because I don't have a test environment and/or a use-case for them. In
>>>> other words, I don't have any "reusable" reserved-memory regions in any
>>>> of the boards (Xilinx and not Xilinx) I have access to. I could add a
>>>> warning if we find a "reusable" reserved-memory region at boot.
>>>
>>> Don't get me wrong, I don't ask for the implementation now, so a warning
>>> would be fine here. However, you need at least to show me some ground that
>>> re-usable memory can be implemented with your solution or they are not a
>>> concern for Xen at all.
>>>
>>>>
>>>> Nonetheless, if you have a concrete suggestion which doesn't require a
>>>> complete rework of this series, I can try to put extra effort to handle
>>>> this case even if it is not a benefit to my employer. I am also open to
>>>> the possibility of dropping patches 6-10 from the series.
>>> I don't think the series as it is would allow us to support re-usable
>>> memory. However as I haven't spent enough time to understand how this could
>>> be possibly dealt. So I am happy to be proved wrong.
>>
>> I thought a bit more about this series during the night. I do agree that we
>> need to improve the support of the reserved-memory today as we may give memory
>> to the allocator that are could be exposed to a guest via a different method
>> (iomem). So carving out the reserved-memory region from the memory allocator
>> is the first step to go.
>>
>> Now we have to differentiate the hardware domain from the other guests. I
>> don't have any objection regarding the way to map reserved-memory region to
>> the hardware domain because this is completely internal to Xen. However, I
>> have some objections with the current interface for DomU:
>>     1) It is still unclear how "reusable" property would fit in that story
>>     2) It is definitely not possible for a user to use 'iomem' for
>> reserved-memory region today because the partial Device-Tree doesn't allow you
>> to create /reserved-memory node nor /memory
>>     3) AFAIK, there are no way for to prevent the hardware domain to use the
>> reserved-region (status = "disabled" would not work).
>> So, IHMO, the guest support for reserved-memory is not in shape. So I think it
>> would be best if we don't permit the reserved-memory region in the iomem
>> rangeset. This would avoid us to tie us in an interface until we figure out
>> the correct plan for guest.
> 
> Wouldn't be proper documentation be enough? (See below for where the
> documentation should live.)
> 
> This is not about privilege over the system: whoever will make the
> decision to ask the hypervisor to map the page will have all the
> necessary rights to do it.  If the user wants to map a given region,
> either because she knows what she is doing, because she is
> experimenting, or for whatever reason, I think she should be allowed. In
> fact, she can always do it by reverting the patch. So why make it
> inconvenient for her?
TBH, I am getting very frustrated on reviewing this series. We spent our 
previous f2f meetings discussing reserved-memory in lengthy way. We also 
agreed on a plan (see below), but now we are back on square one again...

Yes, a user will need to revert the patch. But then as you said the user 
would know what he/she is doing. So reverting a patch is not going to be 
a complication.

However, I already pointed out multiple time that giving permission is 
not going to be enough. So I still don't see the value of having that in 
Xen without an easy way to use it.

For reminder, you agreed on the following splitting the series in 3 parts:
    - Part 1: Extend iomem to support cacheability
    - Part 2: Partially support reserved-memory for Dom0 and don't give 
iomem permission on them
    - Part 3: reserved-memory for guest

I agreed to merge part 1 and 2. Part 3 will be a start for a discussion 
how this should be supported for guest. I also pointed out that Xilinx 
can carry part 3 in their tree if they feel like too.

> 
> 
>> With that in place, I don't have a strong objection with patches 6-10.
>>
>> In any case I think you should clearly spell out in the commit message what
>> kind of reserved-memory region is supported.
> 
> Yes, this makes sense. I am thinking of adding a note to SUPPORT.md. Any
> other places where I should write it down aside from commit messages?
> 
> 
>> For instance, by just going through the binding, I have the feeling
>> that those properties are not actually supported:
>>      1) "no-map" - It is used to tell the OS to not create a virtual memory of
>> the region as part of its standard mapping of system memory, nor permit
>> speculative access to it under any circumstances other than under the control
>> of the device driver using the region. On Arm64, Xen will map reserved-memory
>> as part of xenheap (i.e the direct mapping), but carving out from xenheap
>> would not be sufficient as we use 1GB block for the mapping. So they may still
>> be covered. I would assume this is used for memory that needs to be mapped
>> non-cacheable, so it is potentially critical as Xen would map them cacheable
>> in the stage-1 hypervisor page-tables.
>>      2) "alloc-ranges": it is used to specify regions of memory where it is
>> acceptable to allocate memory from. This may not play well with the Dom0
>> memory allocator.
>>      3) "reusable": I mention here only for completeness. My understanding is
>> it could potentially be used for hypercall buffer. This needs to be
>> investigated.
> 
> Yes, you are right about these properties not being properly supported.
> Do you think that I should list them in SUPPORT.md under a new iomem
> section? Or do you prefer a longer document under docs/? Or both?

The properties have nothing to do with iomem. So it would be clearly the 
wrong place to put under. Instead this should be a separate sections.

Cheers,

-- 
Julien Grall
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