Re: [PATCH v8 13/40] x86/kernel: Make the bss.decrypted section shared in RMP table

Brijesh Singh
2022-01-05T21:39:45Z

On 1/5/22 2:27 PM, Dave Hansen wrote:
> On 1/5/22 11:52, Brijesh Singh wrote:
>>>>           for (; vaddr < vaddr_end; vaddr += PMD_SIZE) {
>>>> +            /*
>>>> +             * When SEV-SNP is active then transition the page to
>>>> shared in the RMP
>>>> +             * table so that it is consistent with the page table
>>>> attribute change.
>>>> +             */
>>>> +            early_snp_set_memory_shared(__pa(vaddr), __pa(vaddr),
>>>> PTRS_PER_PMD);
>>>
>>> Shouldn't the first argument be vaddr as below?
>>
>> Nope, sme_postprocess_startup() is called while we are fixing the
>> initial page table and running with identity mapping (so va == pa).
>
> I'm not sure I've ever seen a line of code that wanted a comment so badly.

The early_snp_set_memory_shared() call the PVALIDATE instruction to
clear the validated bit from the BSS region. The PVALIDATE instruction
needs a virtual address, so we need to use the identity mapped virtual
address so that PVALIDATE can clear the validated bit. I will add more
comments to clarify it.

-Brijesh