Date:   Mon, 20 Mar 2023 17:44:31 +0100
From:   Peter Zijlstra <peterz@infradead.org>
To:     "H. Peter Anvin" <hpa@zytor.com>
Cc:     Lai Jiangshan <jiangshanlai@gmail.com>, Xin Li <xin3.li@intel.com>,
        linux-kernel@vger.kernel.org, x86@kernel.org, kvm@vger.kernel.org,
        tglx@linutronix.de, mingo@redhat.com, bp@alien8.de,
        dave.hansen@linux.intel.com, andrew.cooper3@citrix.com,
        seanjc@google.com, pbonzini@redhat.com, ravi.v.shankar@intel.com
Subject: Re: [PATCH v5 22/34] x86/fred: FRED initialization code
Message-ID: <20230320164431.GQ2194297@hirez.programming.kicks-ass.net>
References: <20230307023946.14516-1-xin3.li@intel.com>
 <20230307023946.14516-23-xin3.li@intel.com>
 <CAJhGHyADXz-3PCFS3M_7TJ8qLGJ=4NcV9aBWrpjemuXB_SnMGg@mail.gmail.com>
 <5D679723-D84F-42F0-AD8A-8BD1A38FB6CD@zytor.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <5D679723-D84F-42F0-AD8A-8BD1A38FB6CD@zytor.com>
Precedence: bulk

On Fri, Mar 17, 2023 at 02:32:28PM -0700, H. Peter Anvin wrote:
> The purpose of separate stacks for NMI, #DB and #MC *in the kernel*
> (remember that user space faults are always taken on stack level 0) is
> to avoid overflowing the kernel stack. #DB in the kernel would imply
> the use of a kernel debugger.

Perf (and through it bpf) also has access to #DB. They can set
breakpoints on kernel instructions/memory just fine provided permission
etc.