Re: Reserved bits and commit x86/sev-es: Set x86_virt_bits to the correct value straight away, instead of a two-phase approach

[Tom Lendacky]

On 1/30/24 15:33, Jacob Xu wrote:
> Adding some AMD folk to the thread here.
>
> For AMD CPUs, initialization of c->x86_phys_bits occurs in
> get_cpu_address_sizes() which is called from early_identify_cpu().
>
> However, early_identify_cpu() will first call early_init_amd() which adjusts
> x86_phys_bits based on the PhysAddrReduction CPUID field.
>
> c->x86_phys_bits -= (cpuid_ebx(0x8000001f) >> 6) & 0x3f;
>
> Thus, this adjustment is ignored.
>
> Adding a new cpu_dev callback to calculate num reserved_cpu_bits makes sense to
> me, hopefully the AMD folk can chime in here though.

Later identify_cpu() calls init_amd() which then makes the adjustment. So 
there is a window between when the value is at 48 and when it gets reduced 
to 43 (on my Milan system).

The actual flow has setup_arch() set the value to MAX_PHYSMEM_BITS, which 
is 46. Then early_detect_mem_encrypt() reduces that to 41. Then 
get_cpu_address_sizes() resets it to 48. Then a bit later, identify_cpu() 
calls init_amd() which calls early_init_amd() which calls 
early_detect_mem_encrypt() which reduces x86_phys_bits to 43.

Looking closer, if mem_encrypt=off is specified, then X86_FEATURE_SME is 
cleared and it is X86_FEATURE_SEV that causes the adjustment. If 
X86_FEATURE_SEV also gets cleared, we won't make the adjustment even 
though when we should.

So I like the idea of a callback to calculate the number of reserved 
physical address bits.

Thanks,
Tom

> Jacob