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

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Message-ID: <Zg3NSI1Max1iHrAI@google.com>
Date: Wed, 3 Apr 2024 21:42:32 +0000
From: Sean Christopherson <seanjc@...gle.com>
To: Kai Huang <kai.huang@...el.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 Thu, Apr 04, 2024, Kai Huang wrote:
> On 4/04/2024 7:57 am, Sean Christopherson wrote:
> > On Wed, Mar 27, 2024, Kai Huang wrote:
> > > 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..

Ya, keeping PAT_* is the only option I strongly object to.  I have no problem
replacing MTRR_TPYE_* usage, I would simply prefer not to do it in this series.
And I obviously have no problem leaving the MTRR stuff as-is.