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Subject: Re: [PATCH 7/7] KVM: arm/arm64: Elide CMOs when unmapping a range
To:     Marc Zyngier <maz@kernel.org>
Cc:     Julien Thierry <julien.thierry.kdev@gmail.com>,
        Suzuki K Poulose <suzuki.poulose@arm.com>,
        James Hogan <jhogan@kernel.org>,
        Paul Mackerras <paulus@ozlabs.org>,
        Paolo Bonzini <pbonzini@redhat.com>,
        =?UTF-8?B?UmFkaW0gS3LEjW3DocWZ?= <rkrcmar@redhat.com>,
        Sean Christopherson <sean.j.christopherson@intel.com>,
        Vitaly Kuznetsov <vkuznets@redhat.com>,
        Wanpeng Li <wanpengli@tencent.com>,
        Jim Mattson <jmattson@google.com>,
        Joerg Roedel <joro@8bytes.org>,
        linux-arm-kernel@lists.infradead.org, kvmarm@lists.cs.columbia.edu,
        linux-mips@vger.kernel.org, kvm-ppc@vger.kernel.org,
        kvm@vger.kernel.org
References: <20191213182503.14460-1-maz@kernel.org>
 <20191213182503.14460-8-maz@kernel.org>
From:   James Morse <james.morse@arm.com>
Message-ID: <0c832b27-7041-a6c8-31c0-d71a25c6f5b8@arm.com>
Date:   Wed, 18 Dec 2019 15:07:48 +0000
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Hi Marc,

On 13/12/2019 18:25, Marc Zyngier wrote:
> If userspace issues a munmap() on a set of pages, there is no
> expectation that the pages are cleaned to the PoC.

(Pedantry: Clean and invalidate. If the guest wrote through a device mapping, we ditch any
clean+stale lines with this path, meaning swapout saves the correct values)


> So let's
> not do more work than strictly necessary, and set the magic
> flag that avoids CMOs in this case.

I think this assumes the pages went from anonymous->free, so no-one cares about the contents.

If the pages are backed by a file, won't dirty pages will still get written back before
the page is free? (e.g. EFI flash 'file' mmap()ed in)

What if this isn't the only mapping of the page? Can't it be swapped out from another VMA?
(tenuous example, poor man's memory mirroring?)


Thanks,

James