linux-kernel - Re: [PATCH 1/1] KVM: VMX: Cleanup VMX basic information defines and usages

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Date:   Thu, 19 Oct 2023 09:15:14 -0700
From:   Sean Christopherson <seanjc@...gle.com>
To:     Xin Li <xin@...or.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        pbonzini@...hat.com, tglx@...utronix.de, mingo@...hat.com,
        bp@...en8.de, dave.hansen@...ux.intel.com, x86@...nel.org,
        hpa@...or.com, weijiang.yang@...el.com
Subject: Re: [PATCH 1/1] KVM: VMX: Cleanup VMX basic information defines and usages

On Thu, Oct 19, 2023, Xin Li wrote:
> On 10/18/2023 2:08 PM, Sean Christopherson wrote:
> 
> > > Add IA32_VMX_BASIC MSR bitfield shift macros and use them to define VMX
> > > basic information bitfields.
> > 
> > Why?  Unless something actually uses the shift independently, just define the
> > BIT_ULL(...) straightaway.
> 
> Well, reading "BIT_ULL(49) | BIT_ULL(54) | BIT_ULL(55) |" is hard.

I wasn't suggesting that, I was suggesting:

 #define VMX_BASIC_INOUT                          BIT_ULL(54)

instead of

 #define VMX_BASIC_INOUT                          BIT_ULL(VMX_BASIC_INOUT_SHIFT)

Defining a shift adds a pointless layer of indirection (if the shift isn't used
directly).  It's especially problematic when there are a series of definitions.
E.g. if I want to know which bit a flag corresponds to, this:

  #define VMX_BASIC_32BIT_PHYS_ADDR_ONLY          BIT_ULL(48)
  #define VMX_BASIC_DUAL_MONITOR_TREATMENT        BIT_ULL(49)
  #define VMX_BASIC_MEM_TYPE(x)                   (((x) & GENMASK_ULL(53, 50)) >> 50)
  #define VMX_BASIC_INOUT                         BIT_ULL(54)
  #define VMX_BASIC_TRUE_CTLS                     BIT_ULL(55)

is much easier for me to process than this

  #define VMX_BASIC_32BIT_PHYS_ADDR_ONLY_SHIFT    48
  #define VMX_BASIC_32BIT_PHYS_ADDR_ONLY          BIT_ULL(VMX_BASIC_32BIT_PHYS_ADDR_ONLY_SHIFT)
  #define VMX_BASIC_DUAL_MONITOR_TREATMENT_SHIFT  49
  #define VMX_BASIC_DUAL_MONITOR_TREATMENT        BIT_ULL(VMX_BASIC_DUAL_MONITOR_TREATMENT_SHIFT)
  #define VMX_BASIC_MEM_TYPE_SHIFT                50
  #define VMX_BASIC_INOUT_SHIFT                   54
  #define VMX_BASIC_INOUT                         BIT_ULL(VMX_BASIC_INOUT_SHIFT)
  #define VMX_BASIC_TRUE_CTLS_SHIFT               55
  #define VMX_BASIC_TRUE_CTLS                     BIT_ULL(VMX_BASIC_TRUE_CTLS_SHIFT)

and the former also tends to work better for IDEs that support peeking at macro
definitions.

> > > ---
> > >   arch/x86/include/asm/msr-index.h       | 31 ++++++++++++++++++++------
> > >   arch/x86/kvm/vmx/nested.c              | 10 +++------
> > >   arch/x86/kvm/vmx/vmx.c                 |  2 +-
> > >   tools/arch/x86/include/asm/msr-index.h | 31 ++++++++++++++++++++------
> > 
> > Please drop the tools/ update, copying kernel headers into tools is a perf tools
> > thing that I want no part of.
> > 
> > https://lore.kernel.org/all/Y8bZ%2FJ98V5i3wG%2Fv@google.com
> 
> why can't we simply remove tools/arch/x86/include/asm/msr-index.h?

That's a question for the tools/perf folks, though I believe the answer is partly
that the perf tooling relies on *exactly* matching kernel-internal structures, and
so tools/perf doesn't want to rely on installed headers.

> > > +#define VMX_BASIC_RESERVED_BITS			\
> > > +	(VMX_BASIC_ALWAYS_0 |			\
> > > +	 VMX_BASIC_RESERVED_RANGE_1 |		\
> > > +	 VMX_BASIC_RESERVED_RANGE_2)
> > 
> > I don't see any value in defining VMX_BASIC_RESERVED_RANGE_1 and
> > VMX_BASIC_RESERVED_RANGE_2 separately.   Or VMX_BASIC_ALWAYS_0 for the matter.
> > And I don't think these macros need to go in msr-index.h, e.g. just define them
> > above vmx_restore_vmx_basic() as that's likely going to be the only user, ever.
> 
> hmm, I'm overusing macros, better do:
> #define VMX_BASIC_RESERVED_BITS			\
> 	(BIT_ULL(31) | GENMASK_ULL(47, 45) | GENMASK_ULL(63, 56))

Please define from high=>low, x86 is little-endian.  I.e.

	 (GENMASK_ULL(63, 56) | GENMASK_ULL(47, 45) | BIT_ULL(31))

> Probably should also move VMX MSR field defs from msr-index.h to
> a vmx header file.

Why bother putting them in a header?  As above, it's extremely unlikely anything
besides vmx_restore_vmx_basic() will ever care about exactly which bits are
reserved.