Re: [Xen-devel] [RFC Patch v4 7/8] x86/hvm: bump the number of pages of shadow memory

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 18/04/18 12:39, Chao Gao wrote:
> On Wed, Apr 18, 2018 at 02:53:03AM -0600, Jan Beulich wrote:
>>>>> On 06.12.17 at 08:50, <chao.gao@xxxxxxxxx> wrote:
>>> Each vcpu of hvm guest consumes at least one shadow page. Currently, only 
>>> 256
>>> (for hap case) pages are pre-allocated as shadow memory at beginning. It 
>>> would
>>> run out if guest has more than 256 vcpus and guest creation fails. Bump the
>>> number of shadow pages to 2 * HVM_MAX_VCPUS for hap case and 8 * 
>>> HVM_MAX_VCPUS
>>> for shadow case.
>>>
>>> This patch won't lead to more memory consumption for the size of shadow 
>>> memory
>>> will be adjusted via XEN_DOMCTL_SHADOW_OP_SET_ALLOCATION according to the 
>>> size
>>> of guest memory and the number of vcpus.
>> I don't understand this: What's the purpose of bumping the values if it 
>> won't lead
>> to higher memory consumption? Afaict there's be higher consumption at least
>> transiently. And I don't see why this would need doing independent of the 
>> intended
>> vCPU count in the guest. I guess you want to base your series on top on 
>> Andrew's
>> max-vCPU-s adjustments (which sadly didn't become ready in time for 4.11).
> The situation here is some pages are pre-allocated as P2M page for domain
> initialization. After vCPU creation, the total number of P2M page are
> adjusted by the domctl interface. Before vCPU creation, this domctl
> is unusable for the check in paging_domctl():
>  if ( unlikely(d->vcpu == NULL) || unlikely(d->vcpu[0] == NULL) )
>
> When the number of a guest's vCPU is small, the pre-allocated are
> enough. But it won't be if the number of vCPU is bigger than 256. Each
> vCPU will at least use one P2M page when it is created, seeing
> contruct_vmcs()->hap_update_paging_modes().

I've also found with XTF tests that the current minimum shadow
calculations are insufficient for single-vcpu domains with 128M of ram.

I haven't had time fix the algorithm, and XTF is using "shadow_memory=4"
as a bodge workaround.

~Andrew

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