Re: [PATCH v3] x86/boot/sev: Avoid shared GHCB page for early memory acceptance

[Tom Lendacky <thomas.lendacky@amd.com>]

On 4/17/25 12:26, Tom Lendacky wrote:
> On 4/17/25 11:38, Ard Biesheuvel wrote:
>> On Thu, 17 Apr 2025 at 18:21, Tom Lendacky <thomas.lendacky@amd.com> wrote:
>>>
>>> On 4/17/25 11:14, Ard Biesheuvel wrote:
>>>> On Thu, 17 Apr 2025 at 18:08, Tom Lendacky <thomas.lendacky@amd.com> wrote:
>>>>>
>>>>> On 4/11/25 14:00, Ard Biesheuvel wrote:
>>>>>> On Fri, 11 Apr 2025 at 20:40, Borislav Petkov <bp@alien8.de> wrote:
>>>>>>>
>>>>>>> On Thu, Apr 10, 2025 at 03:28:51PM +0200, Ard Biesheuvel wrote:
>>>>>>>> From: Ard Biesheuvel <ardb@kernel.org>
>>>>>>>>
>>>>>>>> Communicating with the hypervisor using the shared GHCB page requires
>>>>>>>> clearing the C bit in the mapping of that page. When executing in the
>>>>>>>> context of the EFI boot services, the page tables are owned by the
>>>>>>>> firmware, and this manipulation is not possible.
>>>>>>>>
>>>>>>>> So switch to a different API for accepting memory in SEV-SNP guests, one
>>>>>>>
>>>>>>> That being the GHCB MSR protocol, it seems.
>>>>>>>
>>>>>>
>>>>>> Yes.
>>>>>>
>>>>>>> And since Tom co-developed, I guess we wanna do that.
>>>>>>>
>>>>>>> But then how much slower do we become?
>>>>>>>
>>>>>>
>>>>>> Non-EFI stub boot will become slower if the memory that is used to
>>>>>> decompress the kernel has not been accepted yet. But given how heavily
>>>>>> SEV-SNP depends on EFI boot, this typically only happens on kexec, as
>>>>>> that is the only boot path that goes through the traditional
>>>>>> decompressor.
>>>>>
>>>>> Some quick testing showed no significant differences in kexec booting
>>>>> and testing shows everything seems to be good.
>>>>>
>>>>
>>>> Thanks.
>>>>
>>>>> But, in testing with non-2M sized memory (e.g. a guest with 4097M of
>>>>> memory) and without the change to how SNP is detected before
>>>>> sev_enable() is called, we hit the error path in arch_accept_memory() in
>>>>> arch/x86/boot/compressed/mem.c and the boot crashes.
>>>>>
>>>>
>>>> Right. So this is because sev_snp_enabled() is based on sev_status,
>>>> which has not been set yet at this point, right?
>>>
>>> Correct.
>>>
>>
>> OK. Would this do the trick? (with asm/sev.h added to the #includes)
> 
> Yes, that works for booting. Let me do some kexec testing and get back
> to you. Sorry, that might not be until tomorrow, though.

Ok, found some time... looks good with kexec, too.

Thanks,
Tom

> 
> Thanks,
> Tom
> 
>>
>> --- a/arch/x86/boot/compressed/mem.c
>> +++ b/arch/x86/boot/compressed/mem.c
>> @@ -34,11 +34,14 @@ static bool early_is_tdx_guest(void)
>>
>>  void arch_accept_memory(phys_addr_t start, phys_addr_t end)
>>  {
>> +       static bool sevsnp;
>> +
>>         /* Platform-specific memory-acceptance call goes here */
>>         if (early_is_tdx_guest()) {
>>                 if (!tdx_accept_memory(start, end))
>>                         panic("TDX: Failed to accept memory\n");
>> -       } else if (sev_snp_enabled()) {
>> +       } else if (sevsnp || (sev_get_status() & MSR_AMD64_SEV_SNP_ENABLED)) {
>> +               sevsnp = true;
>>                 snp_accept_memory(start, end);
>>         } else {
>>                 error("Cannot accept memory: unknown platform\n");
>>
>>>>
>>>> And for the record, could you please indicate whether you are ok with
>>>> the co-developed-by/signed-off-by credits on this patch (and
>>>> subsequent revisions)?
>>>
>>> Yep, I'm fine with that.
>>>
>>
>> Thanks.