linux-kernel - Re: [PATCH v6 1/9] x86/cpu: KVM: Add common defines for architectural memory types (PAT, MTRRs, etc.)

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Date: Wed, 27 Mar 2024 11:13:09 +0000
From: "Huang, Kai" <kai.huang@...el.com>
To: "luto@...nel.org" <luto@...nel.org>, "seanjc@...gle.com"
	<seanjc@...gle.com>, "x86@...nel.org" <x86@...nel.org>,
	"dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>,
	"peterz@...radead.org" <peterz@...radead.org>, "bp@...en8.de" <bp@...en8.de>,
	"mingo@...hat.com" <mingo@...hat.com>, "tglx@...utronix.de"
	<tglx@...utronix.de>, "pbonzini@...hat.com" <pbonzini@...hat.com>
CC: "Li, Xin3" <xin3.li@...el.com>, "kvm@...r.kernel.org"
	<kvm@...r.kernel.org>, "Kang, Shan" <shan.kang@...el.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v6 1/9] x86/cpu: KVM: Add common defines for architectural
 memory types (PAT, MTRRs, etc.)

On Fri, 2024-03-08 at 17:27 -0800, Sean Christopherson wrote:
> Add defines for the architectural memory types that can be shoved into
> various MSRs and registers, e.g. MTRRs, PAT, VMX capabilities MSRs, EPTPs,
> etc.  While most MSRs/registers support only a subset of all memory types,
> the values themselves are architectural and identical across all users.
> 
> Leave the goofy MTRR_TYPE_* definitions as-is since they are in a uapi
> header, but add compile-time assertions to connect the dots (and sanity
> check that the msr-index.h values didn't get fat-fingered).
> 
> Keep the VMX_EPTP_MT_* defines so that it's slightly more obvious that the
> EPTP holds a single memory type in 3 of its 64 bits; those bits just
> happen to be 2:0, i.e. don't need to be shifted.
> 
> Opportunistically use X86_MEMTYPE_WB instead of an open coded '6' in
> setup_vmcs_config().
> 
> No functional change intended.
> 
> Signed-off-by: Sean Christopherson <seanjc@...gle.com>
> 

[...]

>  
>  #include "mtrr.h"
>  
> +static_assert(X86_MEMTYPE_UC == MTRR_TYPE_UNCACHABLE);
> +static_assert(X86_MEMTYPE_WC == MTRR_TYPE_WRCOMB);
> +static_assert(X86_MEMTYPE_WT == MTRR_TYPE_WRTHROUGH);
> +static_assert(X86_MEMTYPE_WP == MTRR_TYPE_WRPROT);
> +static_assert(X86_MEMTYPE_WB == MTRR_TYPE_WRBACK);
> +
> 

Hi Sean,

IIUC, the purpose of this patch is for the kernel to use X86_MEMTYPE_xx, which
are architectural values, where applicable?

Yeah we need to keep MTRR_TYPE_xx in the uapi header, but in the kernel, should
we change all places that use MTRR_TYPE_xx to X86_MEMTYPE_xx?  The
static_assert()s above have guaranteed the two are the same, so there's nothing
wrong for the kernel to use X86_MEMTYPE_xx instead.

Both PAT_xx and VMX_BASIC_MEM_TYPE_xx to X86_MEMTYPE_xx, it seems a little bit
odd if we don't switch for MTRR_TYPE_xx.

However by simple search MEM_TYPE_xx are intensively used in many files, so...