linux-kernel - Re: [RFC PATCH V3 13/16] x86/sev: Add Check of #HV event in path

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Date:   Wed, 8 Mar 2023 17:18:13 +0100
From:   "Gupta, Pankaj" <pankaj.gupta@....com>
To:     Tianyu Lan <ltykernel@...il.com>, luto@...nel.org,
        tglx@...utronix.de, mingo@...hat.com, bp@...en8.de,
        dave.hansen@...ux.intel.com, x86@...nel.org, hpa@...or.com,
        seanjc@...gle.com, pbonzini@...hat.com, jgross@...e.com,
        tiala@...rosoft.com, kirill@...temov.name,
        jiangshan.ljs@...group.com, peterz@...radead.org,
        ashish.kalra@....com, srutherford@...gle.com,
        akpm@...ux-foundation.org, anshuman.khandual@....com,
        pawan.kumar.gupta@...ux.intel.com, adrian.hunter@...el.com,
        daniel.sneddon@...ux.intel.com, alexander.shishkin@...ux.intel.com,
        sandipan.das@....com, ray.huang@....com, brijesh.singh@....com,
        michael.roth@....com, thomas.lendacky@....com,
        venu.busireddy@...cle.com, sterritt@...gle.com,
        tony.luck@...el.com, samitolvanen@...gle.com, fenghua.yu@...el.com
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        linux-hyperv@...r.kernel.org, linux-arch@...r.kernel.org
Subject: Re: [RFC PATCH V3 13/16] x86/sev: Add Check of #HV event in path

On 3/1/2023 12:11 PM, Gupta, Pankaj wrote:
> On 1/22/2023 3:46 AM, Tianyu Lan wrote:
>> From: Tianyu Lan <tiala@...rosoft.com>
>>
>> Add check_hv_pending() and check_hv_pending_after_irq() to
>> check queued #HV event when irq is disabled.
>>
>> Signed-off-by: Tianyu Lan <tiala@...rosoft.com>
>> ---
>>   arch/x86/entry/entry_64.S       | 18 +++++++++++++++
>>   arch/x86/include/asm/irqflags.h | 10 +++++++++
>>   arch/x86/kernel/sev.c           | 39 +++++++++++++++++++++++++++++++++
>>   3 files changed, 67 insertions(+)
>>
>> diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
>> index 6baec7653f19..aec8dc4443d1 100644
>> --- a/arch/x86/entry/entry_64.S
>> +++ b/arch/x86/entry/entry_64.S
>> @@ -1064,6 +1064,15 @@ SYM_CODE_END(paranoid_entry)
>>    * R15 - old SPEC_CTRL
>>    */
>>   SYM_CODE_START_LOCAL(paranoid_exit)
>> +#ifdef CONFIG_AMD_MEM_ENCRYPT
>> +    /*
>> +     * If a #HV was delivered during execution and interrupts were
>> +     * disabled, then check if it can be handled before the iret
>> +     * (which may re-enable interrupts).
>> +     */
>> +    mov     %rsp, %rdi
>> +    call    check_hv_pending
>> +#endif
>>       UNWIND_HINT_REGS
>>       /*
>> @@ -1188,6 +1197,15 @@ SYM_CODE_START(error_entry)
>>   SYM_CODE_END(error_entry)
>>   SYM_CODE_START_LOCAL(error_return)
>> +#ifdef CONFIG_AMD_MEM_ENCRYPT
>> +    /*
>> +     * If a #HV was delivered during execution and interrupts were
>> +     * disabled, then check if it can be handled before the iret
>> +     * (which may re-enable interrupts).
>> +     */
>> +    mov     %rsp, %rdi
>> +    call    check_hv_pending
>> +#endif
>>       UNWIND_HINT_REGS
>>       DEBUG_ENTRY_ASSERT_IRQS_OFF
>>       testb    $3, CS(%rsp)
>> diff --git a/arch/x86/include/asm/irqflags.h 
>> b/arch/x86/include/asm/irqflags.h
>> index 7793e52d6237..fe46e59168dd 100644
>> --- a/arch/x86/include/asm/irqflags.h
>> +++ b/arch/x86/include/asm/irqflags.h
>> @@ -14,6 +14,10 @@
>>   /*
>>    * Interrupt control:
>>    */
>> +#ifdef CONFIG_AMD_MEM_ENCRYPT
>> +void check_hv_pending(struct pt_regs *regs);
>> +void check_hv_pending_irq_enable(void);
>> +#endif
>>   /* Declaration required for gcc < 4.9 to prevent 
>> -Werror=missing-prototypes */
>>   extern inline unsigned long native_save_fl(void);
>> @@ -43,12 +47,18 @@ static __always_inline void native_irq_disable(void)
>>   static __always_inline void native_irq_enable(void)
>>   {
>>       asm volatile("sti": : :"memory");
>> +#ifdef CONFIG_AMD_MEM_ENCRYPT
>> +    check_hv_pending_irq_enable();
>> +#endif
>>   }
>>   static inline __cpuidle void native_safe_halt(void)
>>   {
>>       mds_idle_clear_cpu_buffers();
>>       asm volatile("sti; hlt": : :"memory");
>> +#ifdef CONFIG_AMD_MEM_ENCRYPT
>> +    check_hv_pending_irq_enable();
>> +#endif
>>   }
>>   static inline __cpuidle void native_halt(void)
>> diff --git a/arch/x86/kernel/sev.c b/arch/x86/kernel/sev.c
>> index a8862a2eff67..fe5e5e41433d 100644
>> --- a/arch/x86/kernel/sev.c
>> +++ b/arch/x86/kernel/sev.c
>> @@ -179,6 +179,45 @@ void noinstr __sev_es_ist_enter(struct pt_regs 
>> *regs)
>>       this_cpu_write(cpu_tss_rw.x86_tss.ist[IST_INDEX_VC], new_ist);
>>   }
>> +static void do_exc_hv(struct pt_regs *regs)
>> +{
>> +    /* Handle #HV exception. */
>> +}
>> +
>> +void check_hv_pending(struct pt_regs *regs)
>> +{
>> +    if (!cc_platform_has(CC_ATTR_GUEST_SEV_SNP))
>> +        return;
>> +
>> +    if ((regs->flags & X86_EFLAGS_IF) == 0)
>> +        return;
> 
> Will this return and prevent guest from executing NMI's
> while irqs are disabled?

I think we need to handle NMI's even when irqs are disabled.

As we reset "no_further_signal" in hv_raw_handle_exception()
and return from check_hv_pending() when irqs are disabled, this
can result in loss/delay of NMI event?

Thanks,
Pankaj