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

Brijesh Singh
2017-09-03T02:34:48Z

On 9/1/17 10:21 PM, Andy Lutomirski wrote:
> On Fri, Sep 1, 2017 at 3:52 PM, Brijesh Singh <brijesh.singh-5C7GfCeVMHo@public.gmane.org> 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.

Thanks for the suggestion, we could call it DEFINE_PER_CPU_UNENCRYPTED.
I will use it in next rev.

-Brijesh