linux-kernel - Re: [PATCH v6 8/9] KVM: VMX: Open code VMX preemption timer rate mask in its accessor

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Message-ID: <Zipru9eB9oDOOuxf@google.com>
Date: Thu, 25 Apr 2024 07:42:03 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Kai Huang <kai.huang@...el.com>
Cc: Xiaoyao Li <xiaoyao.li@...el.com>, "luto@...nel.org" <luto@...nel.org>, Xin3 Li <xin3.li@...el.com>, 
	"x86@...nel.org" <x86@...nel.org>, 
	"dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>, "peterz@...radead.org" <peterz@...radead.org>, 
	Zhao1 Liu <zhao1.liu@...el.com>, "mingo@...hat.com" <mingo@...hat.com>, 
	"tglx@...utronix.de" <tglx@...utronix.de>, "bp@...en8.de" <bp@...en8.de>, 
	"pbonzini@...hat.com" <pbonzini@...hat.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 8/9] KVM: VMX: Open code VMX preemption timer rate mask
 in its accessor

On Thu, Apr 25, 2024, Kai Huang wrote:
> On Wed, 2024-04-24 at 13:06 -0700, Sean Christopherson wrote:
> > > > static inline u32 vmx_basic_vmcs_mem_type(u64 vmx_basic)
> > > > {
> > > > 	return (vmx_basic & GENMASK_ULL(53, 50)) >>
> > > > 		VMX_BASIC_MEM_TYPE_SHIFT;
> > > > }
> > > > 
> > > > looks not intuitive than original patch.
> > > 
> > > Yeah, agreed, that's taking the worst of both worlds.  I'll update patch 5 to drop
> > > VMX_BASIC_MEM_TYPE_SHIFT when effectively "moving" it into vmx_basic_vmcs_mem_type().
> > 
> > Drat.  Finally getting back to this, dropping VMX_BASIC_MEM_TYPE_SHIFT doesn't
> > work because it's used by nested_vmx_setup_basic(), as is VMX_BASIC_VMCS_SIZE_SHIFT,
> > which is presumably why past me kept them around.
> > 
> > I'm leaning towards keeping things as proposed in this series.  I don't see us
> > gaining a third copy, or even a third user, i.e. I don't think we are creating a
> > future problem by open coding the shift in vmx_basic_vmcs_mem_type().  And IMO
> > code like this
> > 
> > 	return (vmx_basic & VMX_BASIC_MEM_TYPE_MASK) >>
> > 	       VMX_BASIC_MEM_TYPE_SHIFT;
> > 
> > is an unnecessary obfuscation when there is literally one user (the accessor).
> > 
> > Another idea would be to delete VMX_BASIC_MEM_TYPE_SHIFT and VMX_BASIC_VMCS_SIZE_SHIFT,
> > and either open code the values or use local const variables, but that also seems
> > like a net negative, e.g. splits the effective definitions over too many locations.
> 
> Alternatively, we can add macros like below to <asm/vmx.h> close to
> vmx_basic_vmcs_size() etc, so it's straightforward to see.
> 
> +#define VMX_BSAIC_VMCS12_SIZE	((u64)VMCS12_SIZE << 32)
> +#define VMX_BASIC_MEM_TYPE_WB	(MEM_TYPE_WB << 50)

Hmm, it's a bit hard to see it's specifically VMCS12 size, and given that prior
to this series, VMX_BASIC_MEM_TYPE_WB = 6, I'm hesitant to re-introduce/redefine
that macro with a different value.

What if we add a helper in vmx.h to encode the VMCS info?  Then the #defines for
the shifts can go away because the open coded shifts are colocated and more
obviously related.  E.g.

  static inline u64 vmx_basic_encode_vmcs_info(u32 revision, u16 size, u8 memtype)
  {
	return revision | ((u64)size << 32) | ((u64)memtype << 50);
  }


and

  static void nested_vmx_setup_basic(struct nested_vmx_msrs *msrs)
  {
	/*
	 * This MSR reports some information about VMX support. We
	 * should return information about the VMX we emulate for the
	 * guest, and the VMCS structure we give it - not about the
	 * VMX support of the underlying hardware.
	 */
	msrs->basic = vmx_basic_encode_vmcs_info(VMCS12_REVISION, VMCS12_SIZE,
						 X86_MEMTYPE_WB);

	msrs->basic |= VMX_BASIC_TRUE_CTLS
	if (cpu_has_vmx_basic_inout())
		msrs->basic |= VMX_BASIC_INOUT;
  }