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

From: "Jan Beulich" <JBeulich@suse.com>
To: Andrew Cooper <andrew.cooper3@citrix.com>, Chao Gao <chao.gao@intel.com>
Cc: George Dunlap <George.Dunlap@eu.citrix.com>,
	Tim Deegan <tim@xen.org>,
	xen-devel@lists.xen.org
Subject: Re: [RFC Patch v4 7/8] x86/hvm: bump the number of pages of shadow memory
Date: Wed, 18 Apr 2018 05:59:51 -0600	[thread overview]
Message-ID: <5AD7333702000078001BC662@prv1-mh.provo.novell.com> (raw)
In-Reply-To: <20180418113933.GB18077@skl-4s-chao.sh.intel.com>

>>> On 18.04.18 at 13:39, <chao.gao@intel.com> wrote:
> On Wed, Apr 18, 2018 at 02:53:03AM -0600, Jan Beulich wrote:
>>>>> On 06.12.17 at 08:50, <chao.gao@intel.com> 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) )

Hence my reference to Andrew's series.

> 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().

Hmm, that might be a problem perhaps to be addressed in Andrew's series
then, as the implication would be that the amount of shadow/p2m memory
also needs to be set right by XEN_DOMCTL_createdomain (which iirc the
series doesn't do so far).

Jan

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