linux-kernel - Re: [PATCH] riscv: qspinlock: Add virt_spin

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Message-ID: <36jzgc7cpsvphw32wghpegb7t6ezsgd622bm2ws7syjj6mmah2@pameujabd32c>
Date: Mon, 16 Dec 2024 10:47:21 +0100
From: Radim Krčmář <rkrcmar@...tanamicro.com>
To: guoren@...nel.org
Cc: paul.walmsley@...ive.com, palmer@...belt.com, bjorn@...osinc.com, 
	conor@...nel.org, leobras@...hat.com, alexghiti@...osinc.com, atishp@...osinc.com, 
	apatel@...tanamicro.com, linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org, 
	parri.andrea@...il.com, Guo Ren <guoren@...ux.alibaba.com>
Subject: Re: [PATCH] riscv: qspinlock: Add virt_spin_lock() support for KVM
 guests

2024-12-15T11:13:22-0500, guoren@...nel.org:
> Add a static key controlling whether virt_spin_lock() should be
> called or not. When running on bare metal set the new key to
> false.

Wouldn't re-using the combo spinlock qspinlock_key be better?

> The VM guests should fall back to a Test-and-Set spinlock,
> because fair locks have horrible lock 'holder' preemption issues.
> The virt_spin_lock_key would shortcut for the queued_spin_lock_-
> slowpath() function that allow virt_spin_lock to hijack it.

I think we want the proper paravirtualized slowpath, have the
discussions stalled on the SBI side?

Btw. how bad are the performance numbers without this patch?

Thanks.