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

Andy Lutomirski
2017-09-02T03:21:52Z

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.