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

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Date:   Wed, 19 May 2021 15:44:18 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Ashish Kalra <ashish.kalra@....com>,
        Steve Rutherford <srutherford@...gle.com>
Cc:     Borislav Petkov <bp@...en8.de>,
        Sean Christopherson <seanjc@...gle.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>, Joerg Roedel <joro@...tes.org>,
        Tom Lendacky <thomas.lendacky@....com>,
        X86 ML <x86@...nel.org>, KVM list <kvm@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Venu Busireddy <venu.busireddy@...cle.com>,
        Brijesh Singh <brijesh.singh@....com>
Subject: Re: [PATCH v2 2/4] mm: x86: Invoke hypercall when page encryption
 status is changed

On 19/05/21 14:06, Ashish Kalra wrote:
> Now these buffers have very short life and only used for immediate I/O
> and then freed, so they may not be of major concern for SEV
> migration ?

Well, they are a concern because they do break migration.  But it may be 
indeed good enough to just have a WARN ("bad things may happen and you 
get to keep both pieces") and not disable future migration.

A BUG must always be avoided unless you're sure that something *worse* 
will happen in the future, e.g. a BUG is acceptable if you have detected 
a use-after-free or a dangling pointer.  This is not the case.

Paolo

> So disabling migration for failure of address lookup or mapping failures
> on such pages will really be an overkill.
> Might be in favor of Steve's thoughts above of doing a BUG() here
> instead.