linux-kernel - Re: [RFC v2 10/32] x86/tdx: Wire up KVM hypercalls

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Date:   Fri, 7 May 2021 14:46:41 -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,
        Isaku Yamahata <isaku.yamahata@...el.com>
Subject: Re: [RFC v2 10/32] x86/tdx: Wire up KVM hypercalls

On 4/26/21 11:01 AM, Kuppuswamy Sathyanarayanan wrote:
> From: "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
> 
> KVM hypercalls have to be wrapped into vendor-specific TDVMCALLs.

How about:

KVM hypercalls use the "vmcall" or "vmmcall" instructions.  Although the
ABI is similar, those instructions no longer function for TDX guests.
Make TDVMCALLs instead of VMCALL/VMCALL.

> diff --git a/arch/x86/include/asm/kvm_para.h b/arch/x86/include/asm/kvm_para.h
> index 338119852512..2fa85481520b 100644
> --- a/arch/x86/include/asm/kvm_para.h
> +++ b/arch/x86/include/asm/kvm_para.h
> @@ -6,6 +6,7 @@
>  #include <asm/alternative.h>
>  #include <linux/interrupt.h>
>  #include <uapi/asm/kvm_para.h>
> +#include <asm/tdx.h>
>  
>  extern void kvmclock_init(void);
>  
> @@ -34,6 +35,10 @@ static inline bool kvm_check_and_clear_guest_paused(void)
>  static inline long kvm_hypercall0(unsigned int nr)
>  {
>  	long ret;
> +
> +	if (is_tdx_guest())
> +		return tdx_kvm_hypercall0(nr);

... all of these look OK.

>  #endif /* _ASM_X86_TDX_H */
> diff --git a/arch/x86/kernel/tdcall.S b/arch/x86/kernel/tdcall.S
> index 81af70c2acbd..964bfd7fc682 100644
> --- a/arch/x86/kernel/tdcall.S
> +++ b/arch/x86/kernel/tdcall.S
> @@ -11,6 +11,7 @@
>   * refer to TDX GHCI specification).
>   */
>  #define TDVMCALL_EXPOSE_REGS_MASK	0xfc00
> +#define TDVMCALL_VENDOR_KVM		0x4d564b2e584454 /* "TDX.KVM" */
>  
>  /*
>   * TDX guests use the TDCALL instruction to make
> @@ -198,3 +199,9 @@ SYM_FUNC_START(__tdvmcall)
>  	call do_tdvmcall
>  	retq
>  SYM_FUNC_END(__tdvmcall)
> +
> +SYM_FUNC_START(__tdvmcall_vendor_kvm)
> +	movq $TDVMCALL_VENDOR_KVM, %r10
> +	call do_tdvmcall
> +	retq
> +SYM_FUNC_END(__tdvmcall_vendor_kvm)

Granted, this is not a ton of assembly.  But, it does look a bit weird.
 It needs a comment and/or a mention in the changelog.

R10 is not part of the function call ABI, but it is a part of the
TDVMCALL ABI.  This little assembly wrapper lets us reuse do_tdvmcall()
for both KVM-specific hypercalls TDVMCALL_VENDOR_KVM and the more
generic __tdvmcalls.

> --- a/arch/x86/kernel/tdx.c
> +++ b/arch/x86/kernel/tdx.c
> @@ -8,6 +8,10 @@
>  
>  #include <linux/cpu.h>
>  
> +#ifdef CONFIG_KVM_GUEST
> +#include "tdx-kvm.c"
> +#endif
> +
>  static struct {
>  	unsigned int gpa_width;
>  	unsigned long attributes;

I know KVM does weird stuff.  But, this is *really* weird.  Why are we
#including a .c file into another .c file?