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Re: [Xen-devel] Ongoing/future speculative mitigation work

>>> On 26.10.18 at 12:51, <george.dunlap@xxxxxxxxxx> wrote:
> On 10/26/2018 10:56 AM, Jan Beulich wrote:
>>>>> On 26.10.18 at 11:28, <wei.liu2@xxxxxxxxxx> wrote:
>>> On Fri, Oct 26, 2018 at 03:16:15AM -0600, Jan Beulich wrote:
>>>>>>> On 25.10.18 at 18:29, <andrew.cooper3@xxxxxxxxxx> wrote:
>>>>> A split xenheap model means that data pertaining to other guests isn't
>>>>> mapped in the context of this vcpu, so cannot be brought into the cache.
>>>> It was not clear to me from Wei's original mail that talk here is
>>>> about "split" in a sense of "per-domain"; I was assuming the
>>>> CONFIG_SEPARATE_XENHEAP mode instead.
>>> The split heap was indeed referring to CONFIG_SEPARATE_XENHEAP mode, yet
>>> I what I wanted most is the partial direct map which reduces the amount
>>> of data mapped inside Xen context -- the original idea was removing
>>> direct map discussed during one of the calls IIRC. I thought making the
>>> partial direct map mode work and make it as small as possible will get
>>> us 90% there.
>>> The "per-domain" heap is a different work item.
>> But if we mean to go that route, going (back) to the separate
>> Xen heap model seems just like an extra complication to me.
>> Yet I agree that this would remove the need for a fair chunk of
>> the direct map. Otoh a statically partitioned Xen heap would
>> bring back scalability issues which we had specifically meant to
>> get rid of by moving away from that model.
> I think turning SEPARATE_XENHEAP back on would just be the first step.
> We definitely would then need to sort things out so that it's scalable
> again.
> After system set-up, the key difference between xenheap and domheap
> pages is that xenheap pages are assumed to be always mapped (i.e., you
> can keep a pointer to them and it will be valid), whereas domheap pages
> cannot assumed to be mapped, and need to be wrapped with
> [un]map_domain_page().
> The basic solution involves having a xenheap virtual address mapping
> area not tied to the physical layout of the memory.  domheap and xenheap
> memory would have to come from the same pool, but xenheap would need to
> be mapped into the xenheap virtual memory region before being returned.

Wouldn't this most easily be done by making alloc_xenheap_pages()
call alloc_domheap_pages() and then vmap() the result? Of course
we may need to grow the vmap area in that case.


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