Re: [RFC v2 29/32] x86/tdx: Add helper to do MapGPA TDVMALL

[Kuppuswamy, Sathyanarayanan]

On 5/19/21 8:59 AM, Dave Hansen wrote:
> On 4/26/21 11:01 AM, Kuppuswamy Sathyanarayanan wrote:
> > From: "Kirill A. Shutemov" <kirill.shutemov@xxxxxxxxxxxxxxx>
> >
> > MapGPA TDVMCALL requests the host VMM to map a GPA range as private or
> > shared memory mappings. Shared GPA mappings can be used for
> > communication beteen TD guest and host VMM, for example for
> > paravirtualized IO.
>
> As usual, I hate the changelog.  This appears to just be regurgitating
> the spec.
>
> Is this just for part of converting an existing mapping between private
> and shared?  If so, please say that.

How about following change?

    x86/tdx: Add helper to do MapGPA hypercall

    MapGPA hypercall is used by TDX guests to request VMM convert
    the existing mapping of given GPA address range between
    private/shared.

    tdx_hcall_gpa_intent() is the wrapper used for making MapGPA
    hypercall.


> > The new helper tdx_map_gpa() provides access to the operation.
>
> <sigh>  You got your own name wrong. It's tdg_map_gpa() in the patch.

I can use tdx_hcall_gpa_intent().

> BTW, I agree with Sean on this one: "tdg" is a horrible prefix.  You
> just proved Sean's point by mistyping it.  *EVERYONE* is going to rpeat
> that mistake: tdg -> tdx.
>
> > diff --git a/arch/x86/include/asm/tdx.h b/arch/x86/include/asm/tdx.h
> > index dc80cf7f7d08..4789798d7737 100644
> > --- a/arch/x86/include/asm/tdx.h
> > +++ b/arch/x86/include/asm/tdx.h
> > @@ -7,6 +7,11 @@
> >   
> >   #ifndef __ASSEMBLY__
> >   
> > +enum tdx_map_type {
> > +	TDX_MAP_PRIVATE,
> > +	TDX_MAP_SHARED,
> > +};
>
> I like the enum, but please call out that this is a software construct,
> not a part of any hardware or VMM ABI.
>
> >   #ifdef CONFIG_INTEL_TDX_GUEST
> >   
> >   #include <asm/cpufeature.h>
> > @@ -112,6 +117,8 @@ unsigned short tdg_inw(unsigned short port);
> >   unsigned int tdg_inl(unsigned short port);
> >   
> >   extern phys_addr_t tdg_shared_mask(void);
> > +extern int tdg_map_gpa(phys_addr_t gpa, int numpages,
> > +		       enum tdx_map_type map_type);
> >   
> >   #else // !CONFIG_INTEL_TDX_GUEST
> >   
> > @@ -155,6 +162,12 @@ static inline phys_addr_t tdg_shared_mask(void)
> >   {
> >   	return 0;
> >   }
> > +
> > +static inline int tdg_map_gpa(phys_addr_t gpa, int numpages,
> > +			      enum tdx_map_type map_type)
> > +{
> > +	return -ENODEV;
> > +}
>
> FWIW, you could probably get away with just inlining tdg_map_gpa():
>
> static inline int tdg_map_gpa(phys_addr_t gpa, int numpages, ...
> {
> 	u64 ret;
>
> 	if (!IS_ENABLED(CONFIG_INTEL_TDX_GUEST))
> 		return -ENODEV;
>
> 	if (map_type == TDX_MAP_SHARED)
> 		gpa |= tdg_shared_mask();
>
> 	ret = tdvmcall(TDVMCALL_MAP_GPA, gpa, ...
>
> 	return ret ? -EIO : 0;
> }
>
> Then you don't have three copies of the function signature that can get
> out of sync.

I agree that this simplifies the function definition. But, there are
other TDX hypercalls definitions in tdx.c. I can't move all of them to
the header file. If possible, I would like to group all hypercalls in
the same place.

Also, IMO, it is better to hide hypercall internal implementation details
in C file. For example, user of MapGPA hypercall does not care about the
TDVMCALL_MAP_GPA leaf id value. If we inline this function we have to
move such details to header file.


> >   #endif /* CONFIG_INTEL_TDX_GUEST */
> >   #endif /* __ASSEMBLY__ */
> >   #endif /* _ASM_X86_TDX_H */
> > diff --git a/arch/x86/kernel/tdx.c b/arch/x86/kernel/tdx.c
> > index 7e391cd7aa2b..074136473011 100644
> > --- a/arch/x86/kernel/tdx.c
> > +++ b/arch/x86/kernel/tdx.c
> > @@ -15,6 +15,8 @@
> >   #include "tdx-kvm.c"
> >   #endif
> >   
> > +#define TDVMCALL_MAP_GPA	0x10001
> > +
> >   static struct {
> >   	unsigned int gpa_width;
> >   	unsigned long attributes;
> > @@ -98,6 +100,17 @@ static void tdg_get_info(void)
> >   	physical_mask &= ~tdg_shared_mask();
> >   }
> >   
> > +int tdg_map_gpa(phys_addr_t gpa, int numpages, enum tdx_map_type map_type)
> > +{
> > +	u64 ret;
> > +
> > +	if (map_type == TDX_MAP_SHARED)
> > +		gpa |= tdg_shared_mask();
> > +
> > +	ret = tdvmcall(TDVMCALL_MAP_GPA, gpa, PAGE_SIZE * numpages, 0, 0);
> > +	return ret ? -EIO : 0;
> > +}
>
> The naming Intel chose here is nasty.  This doesn't "map" anything.  It
> modifies an existing mapping from what I can tell.  We could name it
> much better than the spec, perhaps:
>
> 	tdx_hcall_gpa_intent()

I will use this function name in next version.

> BTW, all of these hypercalls need a consistent prefix.

I can include _hcall in other hypercall helper functions as well.

> It also needs a comment:
>
> 	/*
> 	 * Inform the VMM of the guest's intent for this physical page:
> 	 * shared with the VMM or private to the guest.  The VMM is
> 	 * expected to change its mapping of the page in response.
> 	 *
> 	 * Note: shared->private conversions require further guest
> 	 * action to accept the page.
> 	 */
>
> The intent here is important.  It makes it clear that this function
> really only plays a role in the conversion process.

Thanks. I will include it in next version.

>

--
Sathyanarayanan Kuppuswamy
Linux Kernel Developer