linux-kernel - Re: [PATCH 10/12] mm: x86: Invoke hypercall when page encryption status is changed

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Date:   Fri, 14 Feb 2020 10:56:53 -0800
From:   Andy Lutomirski <luto@...nel.org>
To:     Ashish Kalra <ashish.kalra@....com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        Radim Krcmar <rkrcmar@...hat.com>,
        Joerg Roedel <joro@...tes.org>, Borislav Petkov <bp@...e.de>,
        Tom Lendacky <thomas.lendacky@....com>,
        David Rientjes <rientjes@...gle.com>, X86 ML <x86@...nel.org>,
        kvm list <kvm@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 10/12] mm: x86: Invoke hypercall when page encryption
 status is changed

On Thu, Feb 13, 2020 at 2:28 PM Ashish Kalra <ashish.kalra@....com> wrote:
>
> On Wed, Feb 12, 2020 at 09:42:02PM -0800, Andy Lutomirski wrote:
> >> On Wed, Feb 12, 2020 at 5:18 PM Ashish Kalra <Ashish.Kalra@....com> wrote:
> >> >
> >> > From: Brijesh Singh <brijesh.singh@....com>
> > >
> > > Invoke a hypercall when a memory region is changed from encrypted ->
> > > decrypted and vice versa. Hypervisor need to know the page encryption
> > > status during the guest migration.
> >>
> >> What happens if the guest memory status doesn't match what the
> >> hypervisor thinks it is?  What happens if the guest gets migrated
> >> between the hypercall and the associated flushes?
>
> This is basically same as the dirty page tracking and logging being done
> during Live Migration. As with dirty page tracking and logging we
> maintain a page encryption bitmap in the kernel which keeps tracks of
> guest's page encrypted/decrypted state changes and this bitmap is
> sync'ed regularly from kernel to qemu and also during the live migration
> process, therefore any dirty pages whose encryption status will change
> during migration, should also have their page status updated when the
> page encryption bitmap is sync'ed.
>
> Also i think that when the amount of dirty pages reach a low threshold,
> QEMU stops the source VM and then transfers all the remaining dirty
> pages, so at that point, there will also be a final sync of the page
> encryption bitmap, there won't be any hypercalls after this as the
> source VM has been stopped and the remaining VM state gets transferred.

And have you ensured that, in the inevitable race when a guest gets
migrated part way through an encryption state change, that no data
corruption occurs?