Re: [Xen-devel] [PATCH] x86/Dom0: account for shadow/HAP allocation

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 26/02/15 07:43, Jan Beulich wrote:
>>>> On 25.02.15 at 18:06, <andrew.cooper3@xxxxxxxxxx> wrote:
>> On 25/02/15 14:45, Jan Beulich wrote:
>>> +static unsigned long __init dom0_paging_pages(const struct domain *d,
>>> +                                              unsigned long nr_pages)
>>> +{
>>> +    /* Copied from: libxl_get_required_shadow_memory() */
>>> +    unsigned long memkb = nr_pages * (PAGE_SIZE / 1024);
>>> +
>>> +    memkb = 4 * (256 * d->max_vcpus + 2 * (memkb / 1024));
>> I have recently raised a bug against Xapi for similar wrong logic when
>> calculating the size of the shadow pool.
>>
>> A per-vcpu reservation of shadow allocation is only needed if shadow
>> paging is actually in use, and even then should match
>> shadow_min_acceptable_pages() at 128 pages per vcpu.
>>
>> If HAP is in use, the only allocations from the shadow pool are for the
>> EPT/NPT tables (1% of nr_pages), IOMMU tables (another 1% of nr_pages if
>> in use), and the logdirty radix tree (substantially less than than 1% of
>> nr_pages).
>>
>> One could argue that structure such as the vmcs/vmcb should have their
>> allocations accounted against the domain, in which case a small per-vcpu
>> component would be appropriate.
>>
>> However as it currently stands, this calculation wastes 4MB of ram per
>> vcpu in shadow allocation which is not going to be used.
> But you realize that the functional change here explicitly only covers
> the shadow case - the PVH (i.e. HAP) case is effectively unchanged
> (merely correcting the mistake of not accounting for what gets
> actually allocated), and I don't intend any functional change for PVH
> (other than said bug fix) with this patch.

Ok

> Hence correcting this (i.e.
> lowering the accounted for as well as the allocated amount) as well
> as adding accounting for VMCS/VMCB (just like we account for
> struct vcpu) should be the subject of a separate patch, presumably
> by someone actively working on PVH (and then perhaps at once for
> libxc). I also think that this calculation would better become a paging
> variant specific hook if calculations differ between shadow and HAP.

That would be better, in the longrun.

~Andrew


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