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

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

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.

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.