linux-kernel - Re: [PATCH v5 08/12] x86/tdx: Add HLT support for TDX guest

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Date:   Tue, 24 Aug 2021 10:35:33 -0700
From:   "Kuppuswamy, Sathyanarayanan" 
        <sathyanarayanan.kuppuswamy@...ux.intel.com>
To:     Borislav Petkov <bp@...en8.de>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Andy Lutomirski <luto@...nel.org>,
        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>,
        x86@...nel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v5 08/12] x86/tdx: Add HLT support for TDX guest



On 8/24/21 9:10 AM, Borislav Petkov wrote:
> On Wed, Aug 04, 2021 at 11:13:25AM -0700, Kuppuswamy Sathyanarayanan wrote:
>> @@ -240,6 +243,32 @@ 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.
> 
> <-- newline in the comment here for better readability.

Ok. I will add it in next version.

> 
>> There leads to significant difference in
> 
> "There leads..." ?

Will fix this in next version. "This leads"

> 
>> +	 * 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,
> 
> s/sti/STI/g

will fix this in next version.

> 
>> +	 * 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
>> +	jne skip_sti
>> +	/* Set R15 register to 0, it is unused in EXIT_REASON_HLT case */
>> +	xor %r15, %r15
>> +	sti
>> +skip_sti:
>>   	tdcall
> 
> ...
> 
>> +static __cpuidle void tdg_safe_halt(void)
>> +{
>> +	u64 ret;
>> +
>> +	/*
>> +	 * Enable interrupts next to the TDVMCALL to avoid
>> +	 * performance degradation.
> 
> That comment needs some more love to say exactly what the problem is.

It is a bug in this submission. After adding STI fix, this local_irq_enable()
had to be removed. Somehow I have missed to do it. I will fix this
in next version.

> 
>> +	 */
>> +	local_irq_enable();
>> +
>> +	/* IRQ is enabled, So set R12 as 0 */
>> +	ret = _tdx_hypercall(EXIT_REASON_HLT, 0, 0, 0, 1, NULL);
>> +
>> +	/* It should never fail */
>> +	BUG_ON(ret);
>> +}
> 

-- 
Sathyanarayanan Kuppuswamy
Linux Kernel Developer