linux-kernel - Re: [PATCH v10 07/11] x86/tdx: Add HLT support for TDX guest

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Date:   Thu, 14 Oct 2021 11:30:45 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Kuppuswamy Sathyanarayanan 
        <sathyanarayanan.kuppuswamy@...ux.intel.com>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        x86@...nel.org, Paolo Bonzini <pbonzini@...hat.com>,
        David Hildenbrand <david@...hat.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Juergen Gross <jgross@...e.com>, Deep Shah <sdeep@...are.com>,
        VMware Inc <pv-drivers@...are.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>
Cc:     Peter H Anvin <hpa@...or.com>, Dave Hansen <dave.hansen@...el.com>,
        Tony Luck <tony.luck@...el.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Andi Kleen <ak@...ux.intel.com>,
        Kirill Shutemov <kirill.shutemov@...ux.intel.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Kuppuswamy Sathyanarayanan <knsathya@...nel.org>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v10 07/11] x86/tdx: Add HLT support for TDX guest

On Fri, Oct 08 2021 at 22:37, Kuppuswamy Sathyanarayanan wrote:
> +/* HLT TDVMCALL sub-function ID */
> +#define EXIT_REASON_HLT			12

arch/x86/include/uapi/asm/vmx.h:#define EXIT_REASON_HLT                 12

Is there a _good_ reason why this can't be reused?

>  /*
>   * __tdx_module_call()  - Helper function used by TDX guests to request
>   * services from the TDX module (does not include VMM services).
> @@ -235,6 +238,33 @@ SYM_FUNC_START(__tdx_hypercall)
>  
>  	movl $TDVMCALL_EXPOSE_REGS_MASK, %ecx
>  
> +	/*
> +	 * For the idle loop STI needs to be called directly before
> +	 * the TDCALL that enters idle (EXIT_REASON_HLT case). STI
> +	 * enables interrupts only one instruction later. If there
> +	 * are any instructions between the STI and the TDCALL for
> +	 * HLT then an interrupt could happen in that time, but the
> +	 * code would go back to sleep afterwards, which can cause
> +	 * longer delays.
> +	 *
> +	 * This leads to significant difference in network performance
> +	 * benchmarks. So add a special case for EXIT_REASON_HLT to
> +	 * trigger STI before TDCALL. But this change is not required
> +	 * for all HLT cases. So use R15 register value to identify the
> +	 * case which needs STI. So, if R11 is EXIT_REASON_HLT and R15
> +	 * is 1, then call STI before TDCALL instruction. Note that R15
> +	 * register is not required by TDCALL ABI when triggering the
> +	 * hypercall for EXIT_REASON_HLT case. So use it in software to
> +	 * select the STI case.
> +	 */
> +	cmpl $EXIT_REASON_HLT, %r11d
> +	jne skip_sti
> +	cmpl $1, %r15d

You already have a nice define for EXIT_REASON_HLT. Please add one for this
constant as well.

> +	jne skip_sti
> +	/* Set R15 register to 0, it is unused in EXIT_REASON_HLT case */
> +	xor %r15, %r15
> +	sti
> +skip_sti:
>  	tdcall

>  bool tdx_get_ve_info(struct ve_info *ve)
>  {
>  	struct tdx_module_output out;
> @@ -84,8 +141,19 @@ bool tdx_get_ve_info(struct ve_info *ve)
>  bool tdx_handle_virtualization_exception(struct pt_regs *regs,
>  					 struct ve_info *ve)
>  {
> -	pr_warn("Unexpected #VE: %lld\n", ve->exit_reason);
> -	return false;
> +	switch (ve->exit_reason) {
> +	case EXIT_REASON_HLT:
> +		tdx_halt();
> +		break;
> +	default:
> +		pr_warn("Unexpected #VE: %lld\n", ve->exit_reason);
> +		return false;
> +	}
> +
> +	/* After successful #VE handling, move the IP */
> +	regs->ip += ve->instr_len;
> +
> +	return true;
>  }
>  
>  void __init tdx_early_init(void)
> @@ -95,5 +163,8 @@ void __init tdx_early_init(void)
>  
>  	setup_force_cpu_cap(X86_FEATURE_TDX_GUEST);
>  
> +	pv_ops.irq.safe_halt = tdx_safe_halt;
> +	pv_ops.irq.halt = tdx_halt;

Colour me confused, but why do we end up in #VE with EXIT_REASON_HLT
when halt/safe_halt is paravirtualized?

There may be a valid reason. If so then this lacks a comment.

Thanks,

        tglx