Re: [patch v3 34/36] x86/smpboot: Implement a bit spinlock to protect the realmode stack

From: Peter Zijlstra <peterz@infradead.org>
To: Thomas Gleixner <tglx@linutronix.de>
Cc: LKML <linux-kernel@vger.kernel.org>,
	x86@kernel.org, David Woodhouse <dwmw2@infradead.org>,
	Andrew Cooper <andrew.cooper3@citrix.com>,
	Brian Gerst <brgerst@gmail.com>,
	Arjan van de Veen <arjan@linux.intel.com>,
	Paolo Bonzini <pbonzini@redhat.com>,
	Paul McKenney <paulmck@kernel.org>,
	Tom Lendacky <thomas.lendacky@amd.com>,
	Sean Christopherson <seanjc@google.com>,
	Oleksandr Natalenko <oleksandr@natalenko.name>,
	Paul Menzel <pmenzel@molgen.mpg.de>,
	"Guilherme G. Piccoli" <gpiccoli@igalia.com>,
	Piotr Gorski <lucjan.lucjanov@gmail.com>,
	Usama Arif <usama.arif@bytedance.com>,
	Juergen Gross <jgross@suse.com>,
	Boris Ostrovsky <boris.ostrovsky@oracle.com>,
	xen-devel@lists.xenproject.org,
	Russell King <linux@armlinux.org.uk>,
	Arnd Bergmann <arnd@arndb.de>,
	linux-arm-kernel@lists.infradead.org,
	Catalin Marinas <catalin.marinas@arm.com>,
	Will Deacon <will@kernel.org>, Guo Ren <guoren@kernel.org>,
	linux-csky@vger.kernel.org,
	Thomas Bogendoerfer <tsbogend@alpha.franken.de>,
	linux-mips@vger.kernel.org,
	"James E.J. Bottomley" <James.Bottomley@hansenpartnership.com>,
	Helge Deller <deller@gmx.de>,
	linux-parisc@vger.kernel.org,
	Paul Walmsley <paul.walmsley@sifive.com>,
	Palmer Dabbelt <palmer@dabbelt.com>,
	linux-riscv@lists.infradead.org,
	Mark Rutland <mark.rutland@arm.com>,
	Sabin Rapan <sabrapan@amazon.com>,
	"Michael Kelley (LINUX)" <mikelley@microsoft.com>,
	David Woodhouse <dwmw@amazon.co.uk>
Subject: Re: [patch v3 34/36] x86/smpboot: Implement a bit spinlock to protect the realmode stack
Date: Tue, 9 May 2023 15:13:40 +0200	[thread overview]
Message-ID: <20230509131340.GA83892@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <20230508185219.123719053@linutronix.de>

On Mon, May 08, 2023 at 09:44:22PM +0200, Thomas Gleixner wrote:

> @@ -252,6 +252,17 @@ SYM_INNER_LABEL(secondary_startup_64_no_
>  	movq	TASK_threadsp(%rax), %rsp
>  
>  	/*
> +	 * Now that this CPU is running on its own stack, drop the realmode
> +	 * protection. For the boot CPU the pointer is NULL!
> +	 */
> +	movq	trampoline_lock(%rip), %rax
	movl	$0, (%rax)

> +.Lsetup_gdt:
> +	/*
>  	 * We must switch to a new descriptor in kernel space for the GDT
>  	 * because soon the kernel won't have access anymore to the userspace
>  	 * addresses where we're currently running on. We have to do that here

> --- a/arch/x86/realmode/rm/trampoline_64.S
> +++ b/arch/x86/realmode/rm/trampoline_64.S
> @@ -37,6 +37,24 @@
>  	.text
>  	.code16
>  
> +.macro LOAD_REALMODE_ESP
> +	/*
> +	 * Make sure only one CPU fiddles with the realmode stack
> +	 */
> +.Llock_rm\@:
> +	btl	$0, tr_lock
> +	jnc	2f
> +	pause
> +	jmp	.Llock_rm\@
> +2:
> +	lock
> +	btsl	$0, tr_lock
> +	jc	.Llock_rm\@

Do we really care about performance here; or should we pick the simpler
form? Also, 'lock' is a prefix, not an instruction.

.Llock_rm\@:
	lock btsl	$0, tr_lock;
	jnc		2f
	pause
	jmp		.Llock_rm\@
2:

> +
> +	# Setup stack
> +	movl	$rm_stack_end, %esp
> +.endm
> +
>  	.balign	PAGE_SIZE