linux-kernel - Re: [RFC PATCH v2 32/32] x86: kvm: Pin the guest memory when SEV is active

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Message-ID: <b2a3cb42-1467-823e-affd-72a0be577932@amd.com>
Date:   Thu, 16 Mar 2017 13:17:47 -0500
From:   Brijesh Singh <brijesh.singh@....com>
To:     Paolo Bonzini <pbonzini@...hat.com>, <simon.guinot@...uanux.org>,
        <linux-efi@...r.kernel.org>, <kvm@...r.kernel.org>,
        <rkrcmar@...hat.com>, <matt@...eblueprint.co.uk>,
        <linux-pci@...r.kernel.org>, <linus.walleij@...aro.org>,
        <gary.hook@....com>, <linux-mm@...ck.org>,
        <paul.gortmaker@...driver.com>, <hpa@...or.com>, <cl@...ux.com>,
        <dan.j.williams@...el.com>, <aarcange@...hat.com>,
        <sfr@...b.auug.org.au>, <andriy.shevchenko@...ux.intel.com>,
        <herbert@...dor.apana.org.au>, <bhe@...hat.com>,
        <xemul@...allels.com>, <joro@...tes.org>, <x86@...nel.org>,
        <peterz@...radead.org>, <piotr.luc@...el.com>, <mingo@...hat.com>,
        <msalter@...hat.com>, <ross.zwisler@...ux.intel.com>, <bp@...e.de>,
        <dyoung@...hat.com>, <thomas.lendacky@....com>, <jroedel@...e.de>,
        <keescook@...omium.org>, <arnd@...db.de>, <toshi.kani@....com>,
        <mathieu.desnoyers@...icios.com>, <luto@...nel.org>,
        <devel@...uxdriverproject.org>, <bhelgaas@...gle.com>,
        <tglx@...utronix.de>, <mchehab@...nel.org>,
        <iamjoonsoo.kim@....com>, <labbott@...oraproject.org>,
        <tony.luck@...el.com>, <alexandre.bounine@....com>,
        <kuleshovmail@...il.com>, <linux-kernel@...r.kernel.org>,
        <mcgrof@...nel.org>, <mst@...hat.com>,
        <linux-crypto@...r.kernel.org>, <tj@...nel.org>,
        <akpm@...ux-foundation.org>, <davem@...emloft.net>
CC:     <brijesh.singh@....com>
Subject: Re: [RFC PATCH v2 32/32] x86: kvm: Pin the guest memory when SEV is
 active



On 03/16/2017 05:38 AM, Paolo Bonzini wrote:
>
>
> On 02/03/2017 16:18, Brijesh Singh wrote:
>> The SEV memory encryption engine uses a tweak such that two identical
>> plaintexts at different location will have a different ciphertexts.
>> So swapping or moving ciphertexts of two pages will not result in
>> plaintexts being swapped. Relocating (or migrating) a physical backing pages
>> for SEV guest will require some additional steps. The current SEV key
>> management spec [1] does not provide commands to swap or migrate (move)
>> ciphertexts. For now we pin the memory allocated for the SEV guest. In
>> future when SEV key management spec provides the commands to support the
>> page migration we can update the KVM code to remove the pinning logical
>> without making any changes into userspace (qemu).
>>
>> The patch pins userspace memory when a new slot is created and unpin the
>> memory when slot is removed.
>>
>> [1] http://support.amd.com/TechDocs/55766_SEV-KM%20API_Spec.pdf
>
> This is not enough, because memory can be hidden temporarily from the
> guest and remapped later.  Think of a PCI BAR that is backed by RAM, or
> also SMRAM.  The pinning must be kept even in that case.
>
> You need to add a pair of KVM_MEMORY_ENCRYPT_OPs (one that doesn't map
> to a PSP operation), such as KVM_REGISTER/UNREGISTER_ENCRYPTED_RAM.  In
> QEMU you can use a RAMBlockNotifier to invoke the ioctls.
>

I was hoping to avoid adding new ioctl, but I see your point. Will add a pair of ioctl's
and use RAMBlocNotifier to invoke those ioctls.

-Brijesh