linux-kernel - RE: [PATCH v10 2/9] KVM: Introduce per-page memory attributes

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Message-ID: <DS0PR11MB63738AE206ADE5EB00D8838BDCF49@DS0PR11MB6373.namprd11.prod.outlook.com>
Date:   Tue, 3 Jan 2023 03:32:53 +0000
From:   "Wang, Wei W" <wei.w.wang@...el.com>
To:     Chao Peng <chao.p.peng@...ux.intel.com>,
        "Qiang, Chenyi" <chenyi.qiang@...el.com>
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Subject: RE: [PATCH v10 2/9] KVM: Introduce per-page memory attributes

On Tuesday, January 3, 2023 9:40 AM, Chao Peng wrote:
> > Because guest memory defaults to private, and now this patch stores
> > the attributes with KVM_MEMORY_ATTRIBUTE_PRIVATE instead of
> _SHARED,
> > it would bring more KVM_EXIT_MEMORY_FAULT exits at the beginning of
> > boot time. Maybe it can be optimized somehow in other places? e.g. set
> > mem attr in advance.
> 
> KVM defaults to 'shared' because this ioctl can also be potentially used by
> normal VMs and 'shared' sounds a value meaningful for both normal VMs and
> confidential VMs. 

Do you mean a normal VM could have pages marked private? What's the usage?
(If all the pages are just marked shared for normal VMs, then why do we need it)

> As for more KVM_EXIT_MEMORY_FAULT exits during the
> booting time, yes, setting all memory to 'private' for confidential VMs through
> this ioctl in userspace before guest launch is an approach for KVM userspace to
> 'override' the KVM default and reduce the number of implicit conversions.

Most pages of a confidential VM are likely to be private pages. It seems more efficient
(and not difficult to check vm_type) to have KVM defaults to "private" for confidential VMs
and defaults to "shared" for normal VMs.