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: <CAMBK9=ZAzKyxuxdsDXNeA8xyPufwFoMxJaHwuh-z+eOmrGD1qQ@mail.gmail.com>
Date:   Wed, 18 Oct 2023 16:02:13 -0700
From:   Adam Dunlap <acdunlap@...gle.com>
To:     "Compostella, Jeremy" <jeremy.compostella@...el.com>
Cc:     Ingo Molnar <mingo@...nel.org>, kirill.shutemov@...ux.intel.com,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "x86@...nel.org" <x86@...nel.org>,
        Felix Held <felix-coreboot@...ixheld.de>
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

On Wed, Oct 18, 2023 at 3:27 PM Compostella, Jeremy
<jeremy.compostella@...el.com> wrote:
> In the light of commit fbf6449f84bf I am wondering what is the right
> approach to fix the regression for AMD and then fix the MTRR check for
> Intel. Should we introduce a new cpu_dev callback to read the number
> of reserved bits and take it into account in get_cpu_address_sizes() ?

I think this approach makes sense. It seems better to have one
function that simply sets it to the right thing rather than setting it
to one value and then adjusting it (fbf6449f84bf did that for
x86_virt_bits, although it caused some other problems). However, I'm
not sure it would solve the problem your original patch tried to fix,
since x86_phys_bits would still be set after intel_init, which
apparently uses the value. Would it work to move the call to
get_cpu_address_sizes() to nearer the start of early_identify_cpu()?
We could also add a cpu_dev callback so it doesn't need the 2-phase
approach, but this would at least bring it back into parity with
v6.6-rc6.

Ex (untested):

---
 arch/x86/kernel/cpu/common.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
index bcd3b2df83bb..cdbe8241e250 100644
--- a/arch/x86/kernel/cpu/common.c
+++ b/arch/x86/kernel/cpu/common.c
@@ -1592,6 +1592,8 @@ static void __init early_identify_cpu(struct
cpuinfo_x86 *c)
  if (!have_cpuid_p())
  identify_cpu_without_cpuid(c);

+ get_cpu_address_sizes(c);
+
  /* cyrix could have cpuid enabled via c_identify()*/
  if (have_cpuid_p()) {
  cpu_detect(c);
@@ -1612,8 +1614,6 @@ static void __init early_identify_cpu(struct
cpuinfo_x86 *c)
  setup_clear_cpu_cap(X86_FEATURE_CPUID);
  }

- get_cpu_address_sizes(c);
-
  setup_force_cpu_cap(X86_FEATURE_ALWAYS);

  cpu_set_bug_bits(c);
-- 

Thanks for finding this problem!
Adam