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

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> 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. 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.

Jan


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