Re: [RFC v1 03/26] x86/cpufeatures: Add is_tdx_guest() interface

[Kuppuswamy, Sathyanarayanan]

On 4/1/21 2:19 PM, Dave Hansen wrote:
> On 4/1/21 2:15 PM, Kuppuswamy, Sathyanarayanan wrote:
> > On 4/1/21 2:08 PM, Dave Hansen wrote:
> > > On 2/5/21 3:38 PM, Kuppuswamy Sathyanarayanan wrote:
> > > > +bool is_tdx_guest(void)
> > > > +{
> > > > +    return static_cpu_has(X86_FEATURE_TDX_GUEST);
> > > > +}
> > >
> > > Why do you need is_tdx_guest() as opposed to calling
> > > cpu_feature_enabled(X86_FEATURE_TDX_GUEST) everywhere?
> >
> > is_tdx_guest() is also implemented/used in compressed
> > code (which uses native_cpuid calls). I don't think
> > we can use cpu_feature_enabled(X86_FEATURE_TDX_GUEST) in
> > compressed code right? Also is_tdx_guest() looks easy
> > to read and use.
>
> OK, but how many of the is_tdx_guest() uses are in the compressed code?
>   Why has its use spread beyond that?
Its only used in handling in/out instructions in compressed code. But this
code shared with in/out handling on non-compressed code.

#define __out(bwl, bw)                                                  \
do {                                                                    \
        if (is_tdx_guest()) {                                           \
                asm volatile("call tdg_out" #bwl : :                    \
                                "a"(value), "d"(port));                 \
        } else {                                                        \
                asm volatile("out" #bwl " %" #bw "0, %w1" : :           \
                                "a"(value), "Nd"(port));                \
        }                                                               \
} while (0)
#define __in(bwl, bw)                                                   \
do {                                                                    \
        if (is_tdx_guest()) {                                           \
                asm volatile("call tdg_in" #bwl :                       \
                                "=a"(value) : "d"(port));               \
        } else {                                                        \
                asm volatile("in" #bwl " %w1, %" #bw "0" :              \
                                "=a"(value) : "Nd"(port));              \
        }                                                               \
} while (0)

>

--
Sathyanarayanan Kuppuswamy
Linux Kernel Developer