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

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Message-ID: <20240130213354.863963-1-jacobhxu@google.com>
Date: Tue, 30 Jan 2024 21:33:54 +0000
From: Jacob Xu <jacobhxu@...gle.com>
To: jeremy.compostella@...el.com
Cc: acdunlap@...gle.com, Thomas.Lendacky@....com, michael.roth@....com, 
	ashish.kalra@....com, bp@...en8.de, felix-coreboot@...ixheld.de, 
	kirill.shutemov@...ux.intel.com, linux-kernel@...r.kernel.org, 
	mingo@...nel.org, x86@...nel.org
Subject: Re: Reserved bits and commit x86/sev-es: Set x86_virt_bits to the
 correct value straight away, instead of a two-phase approach

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.

Jacob