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

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Message-ID: <d139d180-9be7-3f7e-22db-d39fd09dfcb5@intel.com>
Date:   Wed, 19 May 2021 08:59:04 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     Kuppuswamy Sathyanarayanan 
        <sathyanarayanan.kuppuswamy@...ux.intel.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Andy Lutomirski <luto@...nel.org>,
        Dan Williams <dan.j.williams@...el.com>,
        Tony Luck <tony.luck@...el.com>
Cc:     Andi Kleen <ak@...ux.intel.com>,
        Kirill Shutemov <kirill.shutemov@...ux.intel.com>,
        Kuppuswamy Sathyanarayanan <knsathya@...nel.org>,
        Raj Ashok <ashok.raj@...el.com>,
        Sean Christopherson <seanjc@...gle.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [RFC v2 29/32] x86/tdx: Add helper to do MapGPA TDVMALL

On 4/26/21 11:01 AM, Kuppuswamy Sathyanarayanan wrote:
> From: "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
> 
> 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.

> 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.

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.

>  #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()

BTW, all of these hypercalls need a consistent prefix.

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.