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Date:   Wed, 9 Feb 2022 18:05:52 +0000
From:   Sean Christopherson <seanjc@google.com>
To:     "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
Cc:     Thomas Gleixner <tglx@linutronix.de>, bp@alien8.de,
        aarcange@redhat.com, ak@linux.intel.com, dan.j.williams@intel.com,
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        jgross@suse.com, jmattson@google.com, joro@8bytes.org,
        jpoimboe@redhat.com, knsathya@kernel.org,
        linux-kernel@vger.kernel.org, luto@kernel.org, mingo@redhat.com,
        pbonzini@redhat.com, peterz@infradead.org,
        sathyanarayanan.kuppuswamy@linux.intel.com, sdeep@vmware.com,
        tony.luck@intel.com, vkuznets@redhat.com, wanpengli@tencent.com,
        x86@kernel.org
Subject: Re: [PATCHv2.1 05/29] x86/tdx: Add HLT support for TDX guests
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On Wed, Feb 09, 2022, Kirill A. Shutemov wrote:
> On Mon, Feb 07, 2022 at 10:52:19PM +0000, Sean Christopherson wrote:
> .Lskip_sti:
> 	tdcall
> 
> 	/*
> 	 * TDVMCALL leaf does not suppose to fail. If it fails something
> 	 * is horribly wrong with TDX module. Stop the world.
> 	 */
> 	test %rax, %rax
> 	je .Lsuccess
> 	ud2

If the ud2 or call to an external "do panic" helper is out-of-line, then the happy
path avoids a taken branch.  Not a big deal, but it's also trivial to do.

> .Lsuccess:
> 	/* TDVMCALL leaf return code is in R10 */
> 	movq %r10, %rax
> 
> 	/* Copy hypercall result registers to arg struct if needed */
> 	testq $TDX_HCALL_HAS_OUTPUT, %rsi
> 	jz .Lout
> 
> 	movq %r10, TDX_HYPERCALL_r10(%rdi)
> 	movq %r11, TDX_HYPERCALL_r11(%rdi)
> 	movq %r12, TDX_HYPERCALL_r12(%rdi)
> 	movq %r13, TDX_HYPERCALL_r13(%rdi)
> 	movq %r14, TDX_HYPERCALL_r14(%rdi)
> 	movq %r15, TDX_HYPERCALL_r15(%rdi)
> .Lout:
> 	/*
> 	 * Zero out registers exposed to the VMM to avoid
> 	 * speculative execution with VMM-controlled values.
> 	 * This needs to include all registers present in
> 	 * TDVMCALL_EXPOSE_REGS_MASK (except R12-R15).
> 	 * R12-R15 context will be restored.

This comment block should use the "full" 80 chars.

> 	 */
> 	xor %r10d, %r10d
> 	xor %r11d, %r11d
> 
> 	/* Restore callee-saved GPRs as mandated by the x86_64 ABI */
> 	pop %r12
> 	pop %r13
> 	pop %r14
> 	pop %r15
> 
> 	FRAME_END
> 
> 	retq
> SYM_FUNC_END(__tdx_hypercall)
> -- 
>  Kirill A. Shutemov