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From:   Thomas Gleixner <tglx@linutronix.de>
To:     "H. Peter Anvin" <hpa@zytor.com>, Xin Li <xin3.li@intel.com>,
        linux-kernel@vger.kernel.org, x86@kernel.org, kvm@vger.kernel.org
Cc:     mingo@redhat.com, bp@alien8.de, dave.hansen@linux.intel.com,
        peterz@infradead.org, andrew.cooper3@citrix.com, seanjc@google.com,
        pbonzini@redhat.com, ravi.v.shankar@intel.com,
        jiangshanlai@gmail.com, shan.kang@intel.com
Subject: Re: [PATCH v8 31/33] x86/fred: BUG() when ERETU with %rsp not equal
 to that when the ring 3 event was just delivered
In-Reply-To: <c33c4087-eb55-de45-fdb4-b9e0a567cc37@zytor.com>
References: <20230410081438.1750-1-xin3.li@intel.com>
 <20230410081438.1750-32-xin3.li@intel.com> <87ttvm572h.ffs@tglx>
 <c33c4087-eb55-de45-fdb4-b9e0a567cc37@zytor.com>
Date:   Mon, 05 Jun 2023 19:16:30 +0200
Message-ID: <878rcx6d8x.ffs@tglx>
MIME-Version: 1.0
Content-Type: text/plain
Precedence: bulk

On Mon, Jun 05 2023 at 09:42, H. Peter Anvin wrote:
> On 6/5/23 07:15, Thomas Gleixner wrote:
>> On Mon, Apr 10 2023 at 01:14, Xin Li wrote:
>>> A FRED stack frame generated by a ring 3 event should never be messed up, and
>>> the first thing we must make sure is that at the time an ERETU instruction is
>>> executed, %rsp must have the same address as that when the user level event
>>> was just delivered.
>>>
>>> However we don't want to bother the normal code path of ERETU because it's on
>>> the hotest code path, a good choice is to do this check when ERETU
>>> faults.
>> 
>> Which might be not catching bugs where the wrong frame makes ERETU not
>> fault.
>> 
>> We have CONFIG_DEBUG_ENTRY for catching this at the proper place.
>> 
>
> This is true, but this BUG() is a cheap test on a slow path, and thus 
> can be included in production code.

No objection.