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Message-ID: <20230802213930.GB3597003@ls.amr.corp.intel.com>
Date: Wed, 2 Aug 2023 14:39:30 -0700
From: Isaku Yamahata <isaku.yamahata@...il.com>
To: Kai Huang <kai.huang@...el.com>
Cc: peterz@...radead.org, kirill.shutemov@...ux.intel.com,
linux-kernel@...r.kernel.org, dave.hansen@...el.com,
tglx@...utronix.de, bp@...en8.de, mingo@...hat.com, hpa@...or.com,
x86@...nel.org, seanjc@...gle.com, pbonzini@...hat.com,
isaku.yamahata@...el.com,
sathyanarayanan.kuppuswamy@...ux.intel.com,
n.borisov.lkml@...il.com, isaku.yamahata@...il.com
Subject: Re: [PATCH v3 12/12] x86/virt/tdx: Adjust 'struct tdx_module_args'
to use x86 "register index" layout
On Wed, Jul 26, 2023 at 11:25:14PM +1200,
Kai Huang <kai.huang@...el.com> wrote:
> For TDX guest, KVM needs to call __seamcall_saved_ret() to make the
> TDH.VP.ENTER SEAMCALL to enter the guest, possibly taking all registers
> in 'struct tdx_module_args' as input/output.
>
> KVM caches guest's GPRs in 'kvm_vcpu_arch::regs[]', which follows the
> "register index" hardware layout of x86 GPRs. On the other hand, the
> __seamcall_saved_ret() takes the pointer of 'struct tdx_module_args' as
> argument, thus there's a mismatch.
>
> KVM could choose to copy input registers from 'vcpu::regs[]' to a
> 'struct tdx_module_args' and use that as argument to make the SEAMCALL,
> but such memory copy isn't desired and should be avoided if possible.
>
> It's not feasible to change KVM's 'vcpu::regs[]' layout due to various
> reasons (e.g., emulation code uses decoded register index as array index
> to access the register). Therefore, adjust 'struct tdx_module_args' to
> match KVM's 'vcpu::regs[]' layout so that KVM can simply do below:
>
> __seamcall_saved_ret(TDH_VP_ENTER,
> (struct tdx_module_args *)vcpu->arch.regs);
>
> Note RAX/RSP/RBP are not used by the TDX_MODULE_CALL assembly, but they
> are necessary in order match the layout of 'struct tdx_module_args' to
> KVM's 'vcpu::regs[]'. Thus add them to the structure, but name them as
> *_unused. Also don't include them to asm-offset.c so that any misuse of
> them in the assembly would result in build error.
Maybe we can have static check if the offsets match.
e.g. BUILD_BUG_ON(__VCPU_REGS_RAX * 8 != TDX_MODULE_rax); etc...
Anyway, I can have such a patch in TDX KVM side when I use this function for
TDH.VP.ENTER.
Thansk,
>
> Cc: Kirill A. Shutemov <kirill.shutemov@...ux.intel.com>
> Cc: Dave Hansen <dave.hansen@...ux.intel.com>
> Cc: Peter Zijlstra <peterz@...radead.org>
> Cc: Sean Christopherson <seanjc@...gle.com>
> Cc: Paolo Bonzini <pbonzini@...hat.com>
> Cc: Isaku Yamahata <isaku.yamahata@...el.com>
> Suggested-by: Peter Zijlstra <peterz@...radead.org>
> Signed-off-by: Kai Huang <kai.huang@...el.com>
> ---
>
> v2 -> v3:
> - New patch
>
> ---
> arch/x86/include/asm/shared/tdx.h | 19 +++++++++++++------
> arch/x86/kernel/asm-offsets.c | 6 +++---
> 2 files changed, 16 insertions(+), 9 deletions(-)
>
> diff --git a/arch/x86/include/asm/shared/tdx.h b/arch/x86/include/asm/shared/tdx.h
> index 74fc466dfdcd..8d1427562c63 100644
> --- a/arch/x86/include/asm/shared/tdx.h
> +++ b/arch/x86/include/asm/shared/tdx.h
> @@ -58,24 +58,31 @@
> * Used in __tdcall*() to gather the input/output registers' values of the
> * TDCALL instruction when requesting services from the TDX module. This is a
> * software only structure and not part of the TDX module/VMM ABI
> + *
> + * Note those *_unused are not used by the TDX_MODULE_CALL assembly.
> + * The layout of this structure also matches KVM's kvm_vcpu_arch::regs[]
> + * layout, which follows the "register index" order of x86 GPRs. KVM
> + * then can simply type cast kvm_vcpu_arch::regs[] to this structure to
> + * avoid the extra memory copy between two structures when making
> + * TDH.VP.ENTER SEAMCALL.
> */
> struct tdx_module_args {
> - /* callee-clobbered */
> + u64 rax_unused;
> u64 rcx;
> u64 rdx;
> + u64 rbx;
> + u64 rsp_unused;
> + u64 rbp_unused;
> + u64 rsi;
> + u64 rdi;
> u64 r8;
> u64 r9;
> - /* extra callee-clobbered */
> u64 r10;
> u64 r11;
> - /* callee-saved + rdi/rsi */
> u64 r12;
> u64 r13;
> u64 r14;
> u64 r15;
> - u64 rbx;
> - u64 rdi;
> - u64 rsi;
> };
>
> /* Used to communicate with the TDX module */
> diff --git a/arch/x86/kernel/asm-offsets.c b/arch/x86/kernel/asm-offsets.c
> index 6913b372ccf7..e4ad822d3acd 100644
> --- a/arch/x86/kernel/asm-offsets.c
> +++ b/arch/x86/kernel/asm-offsets.c
> @@ -70,6 +70,9 @@ static void __used common(void)
> BLANK();
> OFFSET(TDX_MODULE_rcx, tdx_module_args, rcx);
> OFFSET(TDX_MODULE_rdx, tdx_module_args, rdx);
> + OFFSET(TDX_MODULE_rbx, tdx_module_args, rbx);
> + OFFSET(TDX_MODULE_rsi, tdx_module_args, rsi);
> + OFFSET(TDX_MODULE_rdi, tdx_module_args, rdi);
> OFFSET(TDX_MODULE_r8, tdx_module_args, r8);
> OFFSET(TDX_MODULE_r9, tdx_module_args, r9);
> OFFSET(TDX_MODULE_r10, tdx_module_args, r10);
> @@ -78,9 +81,6 @@ static void __used common(void)
> OFFSET(TDX_MODULE_r13, tdx_module_args, r13);
> OFFSET(TDX_MODULE_r14, tdx_module_args, r14);
> OFFSET(TDX_MODULE_r15, tdx_module_args, r15);
> - OFFSET(TDX_MODULE_rbx, tdx_module_args, rbx);
> - OFFSET(TDX_MODULE_rdi, tdx_module_args, rdi);
> - OFFSET(TDX_MODULE_rsi, tdx_module_args, rsi);
>
> BLANK();
> OFFSET(BP_scratch, boot_params, scratch);
> --
> 2.41.0
>
--
Isaku Yamahata <isaku.yamahata@...il.com>
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