linux-kernel - Re: [PATCH v7 6/6] x86/split_lock: Fix the split lock #AC handling when running as guest

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Date:   Wed, 13 Oct 2021 19:21:26 -0700
From:   Sathyanarayanan Kuppuswamy 
        <sathyanarayanan.kuppuwamy@...ux.intel.com>
To:     Xiaoyao Li <xiaoyao.li@...el.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Kuppuswamy Sathyanarayanan 
        <sathyanarayanan.kuppuswamy@...ux.intel.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        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>,
        "H . Peter 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>,
        Kuppuswamy Sathyanarayanan <knsathya@...nel.org>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v7 6/6] x86/split_lock: Fix the split lock #AC handling
 when running as guest


On 10/13/21 6:24 PM, Xiaoyao Li wrote:
> On 10/14/2021 5:32 AM, Sathyanarayanan Kuppuswamy wrote:
>> + Xiaoyao
>>
>> On 10/13/21 1:30 PM, Sean Christopherson wrote:
>>> On Tue, Oct 05, 2021, Kuppuswamy Sathyanarayanan wrote:
>>>> From: Xiaoyao Li <xiaoyao.li@...el.com>
>>>>
>>>> If running as guest and hypervisor enables
>>>> MSR_TEST_CTRL.SPLIT_LOCK_DETECT during its running, it can get split
>>>> lock #AC even though sld_state is sld_off.
>>> That's a hypervisor bug, no?  The hypervisor should never inject a 
>>> fault that
>>> the guest cannot reasonably expect.
>
> What if hypervisor doesn't intercept #AC and host enables 
> SPLIT_LOCK_DETECT during guest running? That's exactly the case TDX is 
> facing.
>
> (BTW, this patch is not complete that no matter what state sld_state 
> is, it should always treat it as fatal even sld_warn because sld_warn 
> doesn't guarantee SPLIT_LOCK_DETECT is available)
>
> Sathya, we need to drop this one. Andi has anther one to disable split 
> lock detection for SPR. That's a better direction.

Ok. I will fix this in next submission.

>
>>>> For kernel mode #AC, it always dies("split lock"), no more action
>>>> needed.
>>>>
>>>> For user mode #AC, it should treat sld_off (default state when feature
>>>> is not available) as fatal as well.
>>>>
>>>> Signed-off-by: Xiaoyao Li <xiaoyao.li@...el.com>
>>>> Signed-off-by: Kuppuswamy Sathyanarayanan 
>>>> <sathyanarayanan.kuppuswamy@...ux.intel.com>
>>>> ---
>>>>   arch/x86/kernel/cpu/intel.c | 7 ++++++-
>>>>   1 file changed, 6 insertions(+), 1 deletion(-)
>>>>
>>>> diff --git a/arch/x86/kernel/cpu/intel.c b/arch/x86/kernel/cpu/intel.c
>>>> index 01d7935feaed..47f0bc95ce2a 100644
>>>> --- a/arch/x86/kernel/cpu/intel.c
>>>> +++ b/arch/x86/kernel/cpu/intel.c
>>>> @@ -1190,7 +1190,12 @@ static void bus_lock_init(void)
>>>>   bool handle_user_split_lock(struct pt_regs *regs, long error_code)
>>>>   {
>>>> -    if ((regs->flags & X86_EFLAGS_AC) || sld_state == sld_fatal)
>>>> +    /*
>>>> +     * In virtualization environment, it can get split lock #AC 
>>>> even when
>>>> +     * sld_off but hypervisor enables it.
>>>> +     * Thus only handles when sld_warn explicitly.
>>>> +     */
>>>> +    if ((regs->flags & X86_EFLAGS_AC) || sld_state != sld_warn)
>>>>           return false;
>>>>       split_lock_warn(regs->ip);
>>>>       return true;
>>>> -- 
>>>> 2.25.1
>>>>
>
-- 
Sathyanarayanan Kuppuswamy
Linux Kernel Developer