Re: [RFC Design Doc] Add vNVDIMM support for Xen

["Jan Beulich" <JBeulich@suse.com>]

>>> On 17.03.16 at 13:44, <haozhong.zhang@intel.com> wrote:
> On 03/17/16 05:04, Jan Beulich wrote:
>> >>> On 17.03.16 at 09:58, <haozhong.zhang@intel.com> wrote:
>> > On 03/16/16 09:23, Jan Beulich wrote:
>> >> >>> On 16.03.16 at 15:55, <haozhong.zhang@intel.com> wrote:
>> >> > On 03/16/16 08:23, Jan Beulich wrote:
>> >> >> >>> On 16.03.16 at 14:55, <haozhong.zhang@intel.com> wrote:
>> >> >> > On 03/16/16 07:16, Jan Beulich wrote:
>> >> >> >> And
>> >> >> >> talking of fragmentation - how do you mean to track guest
>> >> >> >> permissions for an unbounded number of address ranges?
>> >> >> >>
>> >> >> > 
>> >> >> > In this case range structs in iomem_caps for NVDIMMs may consume a lot
>> >> >> > of memory, so I think they are another candidate that should be put in
>> >> >> > the reserved area on NVDIMM. If we only allow to grant access
>> >> >> > permissions to NVDIMM page by page (rather than byte), the number of
>> >> >> > range structs for each NVDIMM in the worst case is still decidable.
>> >> >> 
>> >> >> Of course the permission granularity is going to by pages, not
>> >> >> bytes (or else we couldn't allow the pages to be mapped into
>> >> >> guest address space). And the limit on the per-domain range
>> >> >> sets isn't going to be allowed to be bumped significantly, at
>> >> >> least not for any of the existing ones (or else you'd have to
>> >> >> prove such bumping can't be abused).
>> >> > 
>> >> > What is that limit? the total number of range structs in per-domain
>> >> > range sets? I must miss something when looking through 'case
>> >> > XEN_DOMCTL_iomem_permission' of do_domctl() and didn't find that
>> >> > limit, unless it means alloc_range() will fail when there are lots of
>> >> > range structs.
>> >> 
>> >> Oh, I'm sorry, that was a different set of range sets I was
>> >> thinking about. But note that excessive creation of ranges
>> >> through XEN_DOMCTL_iomem_permission is not a security issue
>> >> just because of XSA-77, i.e. we'd still not knowingly allow a
>> >> severe increase here.
>> >>
>> > 
>> > I didn't notice that multiple domains can all have access permission
>> > to an iomem range, i.e. there can be multiple range structs for a
>> > single iomem range. If range structs for NVDIMM are put on NVDIMM,
>> > then there would be still a huge amount of them on NVDIMM in the worst
>> > case (maximum number of domains * number of NVDIMM pages).
>> > 
>> > A workaround is to only allow a range of NVDIMM pages be accessed by a
>> > single domain. Whenever we add the access permission of NVDIMM pages
>> > to a domain, we also remove the permission from its current
>> > grantee. In this way, we only need to put 'number of NVDIMM pages'
>> > range structs on NVDIMM in the worst case.
>> 
>> But will this work? There's a reason multiple domains are permitted
>> access: The domain running qemu for the guest, for example,
>> needs to be able to access guest memory.
>>
> 
> QEMU now only maintains ACPI tables and emulates _DSM for vNVDIMM
> which both do not need to access NVDIMM pages mapped to guest.

For one - this was only an example. And then - iirc qemu keeps
mappings of certain guest RAM ranges. If I'm remembering this
right, then why would it be excluded that it also may need
mappings of guest NVDIMM?

>> No matter how much you and others are opposed to this, I can't
>> help myself thinking that PMEM regions should be treated like RAM
>> (and hence be under full control of Xen), whereas PBLK regions
>> could indeed be treated like MMIO (and hence partly be under the
>> control of Dom0).
>>
> 
> Hmm, making Xen has full control could at least make reserving space
> on NVDIMM easier. I guess full control does not include manipulating
> file systems on NVDIMM which can be still left to dom0?
> 
> Then there is another problem (which also exists in the current
> design): does Xen need to emulate NVDIMM _DSM for dom0? Take the _DSM
> that access label storage area (for namespace) for example:
> 
> The way Linux reserving space on pmem mode NVDIMM is to leave the
> reserved space at the beginning of pmem mode NVDIMM and create a pmem
> namespace which starts from the end of the reserved space. Because the
> reservation information is written in the namespace in the NVDIMM
> label storage area, every OS that follows the namespace spec would not
> mistakenly write files in the reserved area. I prefer to the same way
> if Xen is going to do the reservation. We definitely don't want dom0
> to break the label storage area, so Xen seemingly needs to emulate the
> corresponding _DSM functions for dom0? If so, which part, the
> hypervisor or the toolstack, should do the emulation?

I don't think I can answer all but the very last point: Of course this
can't be done in the tool stack, since afaict the Dom0 kernel will
want to evaluate _DSM before the tool stack even runs.

Jan

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