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Subject: Re: [edk2-devel] Question about the OVMF and MTRRs?
To: devel@edk2.groups.io, afish@apple.com
References: <369C9B43-A02C-4AEB-8F02-735E1042ABB2@apple.com>
From: "Laszlo Ersek" <lersek@redhat.com>
Message-ID: <236d9a8e-7c7c-90a5-ca79-849ba6e7f4fb@redhat.com>
Date: Tue, 7 Jul 2020 21:08:52 +0200
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Hi Andrew,

On 07/03/20 19:22, Andrew Fish via groups.io wrote:
> I noticed for my version of QEMU that MTRR registers are not supported. Is that expected? Or is it something if you get wrong it does not break in QEMU?
> 
> Due to no MTRR support the CPU Protocol SetMemoeryAttributes() call is falling [1] and non of the GCD ranges have EFI_MEMORY_UC Attribute. 
> 
> I’m not sure but I think the no MTRR case should attempt to set the attributes via page tables via:
> 
> return AssignMemoryPageAttributes (NULL, BaseAddress, Length, MemoryAttributes, NULL);
> 
> Vs 
> 
> return EFI_UNSUPPORTED;
> 
> [1] https://github.com/tianocore/edk2/blob/master/UefiCpuPkg/CpuDxe/CpuDxe.c#L429

sorry, reaching this email only now.

I wouldn't really expect IsMtrrSupported() to return FALSE.

BOOLEAN
EFIAPI
IsMtrrSupported (
  VOID
  )
{
  CPUID_VERSION_INFO_EDX    Edx;
  MSR_IA32_MTRRCAP_REGISTER MtrrCap;

  //
  // Check CPUID(1).EDX[12] for MTRR capability
  //
  AsmCpuid (CPUID_VERSION_INFO, NULL, NULL, NULL, &Edx.Uint32);
  if (Edx.Bits.MTRR == 0) {
    return FALSE;
  }

  //
  // Check number of variable MTRRs and fixed MTRRs existence.
  // If number of variable MTRRs is zero, or fixed MTRRs do not
  // exist, return false.
  //
  MtrrCap.Uint64 = AsmReadMsr64 (MSR_IA32_MTRRCAP);
  if ((MtrrCap.Bits.VCNT == 0) || (MtrrCap.Bits.FIX == 0)) {
    return FALSE;
  }
  return TRUE;
}


In QEMU, the rdmsr is handled in "target/i386/misc_helper.c", function
helper_rdmsr():

    case MSR_MTRRcap:
        if (env->features[FEAT_1_EDX] & CPUID_MTRR) {
            val = MSR_MTRRcap_VCNT | MSR_MTRRcap_FIXRANGE_SUPPORT |
                MSR_MTRRcap_WC_SUPPORTED;
        } else {
            /* XXX: exception? */
            val = 0;
        }
        break;

thus the second "FALSE" branch in IsMtrrSupported() can never be
reached. (Either the first "FALSE" branch is taken, or the "TRUE" branch
is taken.) MSR_MTRRcap_VCNT is 8.

Regarding the first "FALSE" branch, whether that is taken depends on the
QEMU CPU model (or the QEMU command line). Check "target/i386/cpu.c" for
"CPUID_MTRR".

Unfortunately I can't think of an easy way to list the CPU models that
do *not* have CPUID_MTRR. What is your CPU model?

Thanks
Laszlo