Re: [RFC Part1 PATCH v3 16/17] X86/KVM: Provide support to create Guest and HV shared per-CPU variables

From: Andy Lutomirski <luto@kernel.org>
To: Brijesh Singh <brijesh.singh@amd.com>
Cc: "Borislav Petkov" <bp@suse.de>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	"X86 ML" <x86@kernel.org>,
	"linux-efi@vger.kernel.org" <linux-efi@vger.kernel.org>,
	linuxppc-dev <linuxppc-dev@lists.ozlabs.org>,
	"kvm list" <kvm@vger.kernel.org>,
	"Thomas Gleixner" <tglx@linutronix.de>,
	"Ingo Molnar" <mingo@redhat.com>,
	"H . Peter Anvin" <hpa@zytor.com>,
	"Andy Lutomirski" <luto@kernel.org>,
	"Tony Luck" <tony.luck@intel.com>,
	"Piotr Luc" <piotr.luc@intel.com>,
	"Tom Lendacky" <thomas.lendacky@amd.com>,
	"Fenghua Yu" <fenghua.yu@intel.com>,
	"Lu Baolu" <baolu.lu@linux.intel.com>,
	"Reza Arbab" <arbab@linux.vnet.ibm.com>,
	"David Howells" <dhowells@redhat.com>,
	"Matt Fleming" <matt@codeblueprint.co.uk>,
	"Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>,
	"Laura Abbott" <labbott@redhat.com>,
	"Ard Biesheuvel" <ard.biesheuvel@linaro.org>,
	"Andrew Morton" <akpm@linux-foundation.org>,
	"Eric Biederman" <ebiederm@xmission.com>,
	"Benjamin Herrenschmidt" <benh@kernel.crashing.org>,
	"Paul Mackerras" <paulus@samba.org>,
	"Konrad Rzeszutek Wilk" <konrad.wilk@oracle.com>,
	"Jonathan Corbet" <corbet@lwn.net>,
	"Dave Airlie" <airlied@redhat.com>,
	"Kees Cook" <keescook@chromium.org>,
	"Paolo Bonzini" <pbonzini@redhat.com>,
	"Radim Krčmář" <rkrcmar@redhat.com>,
	"Arnd Bergmann" <arnd@arndb.de>, "Tejun Heo" <tj@kernel.org>,
	"Christoph Lameter" <cl@linux.com>
Subject: Re: [RFC Part1 PATCH v3 16/17] X86/KVM: Provide support to create Guest and HV shared per-CPU variables
Date: Fri, 1 Sep 2017 20:21:52 -0700	[thread overview]
Message-ID: <CALCETrV+rv=9Rg5V1z8vHtVDW64eCNtZHQMW8DipRADvm+qP5A@mail.gmail.com> (raw)
In-Reply-To: <8155b5b2-b2b3-bc8f-33ae-b81b661a2e38@amd.com>

On Fri, Sep 1, 2017 at 3:52 PM, Brijesh Singh <brijesh.singh@amd.com> wrote:
> Hi Boris,
>
> On 08/30/2017 12:46 PM, Borislav Petkov wrote:
>>
>> On Wed, Aug 30, 2017 at 11:18:42AM -0500, Brijesh Singh wrote:
>>>
>>> I was trying to avoid mixing early and no-early set_memory_decrypted()
>>> but if
>>> feedback is: use early_set_memory_decrypted() only if its required
>>> otherwise
>>> use set_memory_decrypted() then I can improve the logic in next rev.
>>> thanks
>>
>>
>> Yes, I think you should use the early versions when you're, well,
>> *early* :-) But get rid of that for_each_possible_cpu() and do it only
>> on the current CPU, as this is a per-CPU path anyway. If you need to
>> do it on *every* CPU and very early, then you need a separate function
>> which is called in kvm_smp_prepare_boot_cpu() as there you're pre-SMP.
>>
>
> I am trying to implement your feedback and now remember why I choose to
> use early_set_memory_decrypted() and for_each_possible_cpu loop. These
> percpu variables are static. Hence before clearing the C-bit we must
> perform the in-place decryption so that original assignment is preserved
> after we change the C-bit. Tom's SME patch [1] added sme_early_decrypt()
> -- which can be used to perform the in-place decryption but we do not have
> similar routine for non-early cases. In order to address your feedback,
> we have to add similar functions. So far, we have not seen the need for
> having such functions except this cases. The approach we have right now
> works just fine and not sure if its worth adding new functions.
>
> Thoughts ?
>
> [1] Commit :7f8b7e7 x86/mm: Add support for early encryption/decryption of
> memory

Shouldn't this be called DEFINE_PER_CPU_UNENCRYPTED?  ISTM the "HV
shared" bit is incidental.