linux-kernel - Re: [PATCH v2 2/4] mm: x86: Invoke hypercall when page encryption status is changed

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Date:   Fri, 14 May 2021 12:38:27 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     Ashish Kalra <ashish.kalra@....com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>, seanjc@...gle.com,
        tglx@...utronix.de, mingo@...hat.com, hpa@...or.com,
        joro@...tes.org, thomas.lendacky@....com, x86@...nel.org,
        kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        srutherford@...gle.com, venu.busireddy@...cle.com,
        brijesh.singh@....com
Subject: Re: [PATCH v2 2/4] mm: x86: Invoke hypercall when page encryption
 status is changed

On Fri, May 14, 2021 at 10:05:19AM +0000, Ashish Kalra wrote:
> No, actually notify_addr_enc_status_changed() is called whenever a range
> of memory is marked as encrypted or decrypted, so it has nothing to do
> with migration as such. 
> 
> This is basically modifying the encryption attributes on the page tables
> and correspondingly also making the hypercall to inform the hypervisor about
> page status encryption changes. The hypervisor will use this information
> during an ongoing or future migration, so this information is maintained
> even though migration might never be initiated here.

Doh, ofcourse. This doesn't make it easier.

> The error value cannot be propogated up the callchain directly
> here,

Yeah, my thinking was way wrong here - sorry about that.

> but one possibility is to leverage the hypercall and use Sean's
> proposed hypercall interface to notify the host/hypervisor to block/stop
> any future/ongoing migration.
>
> Or as from Paolo's response, writing 0 to MIGRATION_CONTROL MSR seems
> more ideal.

Ok.

So to sum up: notify_addr_enc_status_changed() should warn but not
because of migration but because regardless, we should tell the users
when page enc attributes updating fails as that is potentially hinting
at a bigger problem so we better make sufficient noise here.

Thx.

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette