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: Thu, 4 Apr 2024 10:17:17 +1300
From: "Huang, Kai" <kai.huang@...el.com>
To: Sean Christopherson <seanjc@...gle.com>
CC: "luto@...nel.org" <luto@...nel.org>, "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>, Xin3 Li
	<xin3.li@...el.com>, "kvm@...r.kernel.org" <kvm@...r.kernel.org>, Shan Kang
	<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 4/04/2024 7:57 am, Sean Christopherson wrote:
> On Wed, Mar 27, 2024, Kai Huang wrote:
>> 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?
> 
> Maybe?  Probably?
> 
>> 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...
> 
> Yeah, I definitely don't want to do it in this series due to the amount of churn
> that would be required.
> 
>    $ git grep MTRR_TYPE_ | wc -l
>    100
> 
> I'm not even entirely convinced that it would be a net positive.  Much of the KVM
> usage that's being cleaned up is flat out wrong, e.g. using "MTRR" enums in places
> that having nothing to do with MTRRs.  But the majority of the remaining usage is
> in MTRR code, i.e. isn't wrong, and is arguably better off using the MTRR specific
> #defines.

Yeah understood.

But the patch title says we also "add common defines for ... MTRRs", so 
to me looks we should get rid of MTRR_TPYE_xx and use the common ones 
instead.  And it also looks a little bit inconsistent if we remove the 
PAT_xx but keep the MTRR_TYPE_xx.

Perhaps we can keep PAT_xx but add macros?

   #define PAT_UC	X86_MEMTYPE_UC
   ...

But looks not nice either because the only purpose is to keep the PAT_xx..

So up to you :-)