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

Tom Lendacky
2022-01-06T19:06:24Z

On 1/6/22 11:40 AM, Venu Busireddy wrote:
> On 2022-01-05 15:39:22 -0600, Brijesh Singh wrote:
>>
>>
>> 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.
>
> Looking forward to see your final comments explaining this. I can't
> still follow why, when PVALIDATE needs the virtual address, we are doing
> a __pa() on the vaddr.

It's because of the phase of booting that the kernel is in. At this point,
the kernel is running in identity mapped mode (VA == PA). The
__start_bss_decrypted address is a regular kernel address, e.g. for the
kernel I'm on it is 0xffffffffa7600000. Since the PVALIDATE instruction
requires a valid virtual address, the code needs to perform a __pa()
against __start_bss_decrypted to get the identity mapped virtual address
that is currently in place.

It is not until the .Ljump_to_C_code label in head_64.S that the
addressing switches from identity mapped to kernel addresses.

Thanks,
Tom

>
> Venu
>