Re: [PATCH 0/5] Drivers: hv: Fix NEED_RESCHED_LAZY and use common APIs

From: Wei Liu <wei.liu@kernel.org>
To: Sean Christopherson <seanjc@google.com>
Cc: Nuno Das Neves <nunodasneves@linux.microsoft.com>,
	Marc Zyngier <maz@kernel.org>,
	Oliver Upton <oliver.upton@linux.dev>,
	Catalin Marinas <catalin.marinas@arm.com>,
	Will Deacon <will@kernel.org>,
	Tianrui Zhao <zhaotianrui@loongson.cn>,
	Bibo Mao <maobibo@loongson.cn>,
	Huacai Chen <chenhuacai@kernel.org>,
	Anup Patel <anup@brainfault.org>,
	Paul Walmsley <paul.walmsley@sifive.com>,
	Palmer Dabbelt <palmer@dabbelt.com>,
	Albert Ou <aou@eecs.berkeley.edu>,
	Paolo Bonzini <pbonzini@redhat.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
	Dave Hansen <dave.hansen@linux.intel.com>,
	x86@kernel.org, "K. Y. Srinivasan" <kys@microsoft.com>,
	Haiyang Zhang <haiyangz@microsoft.com>,
	Wei Liu <wei.liu@kernel.org>, Dexuan Cui <decui@microsoft.com>,
	Peter Zijlstra <peterz@infradead.org>,
	Andy Lutomirski <luto@kernel.org>,
	"Paul E. McKenney" <paulmck@kernel.org>,
	Frederic Weisbecker <frederic@kernel.org>,
	Neeraj Upadhyay <neeraj.upadhyay@kernel.org>,
	Joel Fernandes <joelagnelf@nvidia.com>,
	Josh Triplett <josh@joshtriplett.org>,
	Boqun Feng <boqun.feng@gmail.com>,
	Uladzislau Rezki <urezki@gmail.com>,
	linux-kernel@vger.kernel.org,
	linux-arm-kernel@lists.infradead.org, kvmarm@lists.linux.dev,
	kvm@vger.kernel.org, loongarch@lists.linux.dev,
	kvm-riscv@lists.infradead.org, linux-riscv@lists.infradead.org,
	linux-hyperv@vger.kernel.org, rcu@vger.kernel.org
Subject: Re: [PATCH 0/5] Drivers: hv: Fix NEED_RESCHED_LAZY and use common APIs
Date: Tue, 26 Aug 2025 16:58:00 +0000	[thread overview]
Message-ID: <aK3nmOgVl4INJpjG@liuwe-devbox-ubuntu-v2.tail21d00.ts.net> (raw)
In-Reply-To: <aKz_ZMvvF0e9nwSn@google.com>

On Mon, Aug 25, 2025 at 05:27:16PM -0700, Sean Christopherson wrote:
> On Mon, Aug 25, 2025, Nuno Das Neves wrote:
> > On 8/25/2025 1:06 PM, Sean Christopherson wrote:
> > > Fix a bug where MSHV root partitions don't honor NEED_RESCHED_LAZY, and then
> > > deduplicate the TIF related MSHV code by turning the "kvm" entry APIs into
> > > more generic "virt" APIs (which ideally would have been done when MSHV root
> > > support was added).
> > > 
> > > Assuming all is well, maybe this could go through the tip tree?
> > > 
> > > The Hyper-V stuff and non-x86 architectures are compile-tested only.
> > > 
> > 
> > Thanks Sean, I can test the root partition changes.
> > 
> > A similar change will be needed in mshv_vtl_main.c since it also calls
> > mshv_do_pre_guest_mode_work() (hence the "common" in mshv_common.c).
> 
> Oof, more dependencies.  I suppose the easiest thing would be to send a series
> against
> 
>   git://git.kernel.org/pub/scm/linux/kernel/git/hyperv/linux.git queue
> 
> and then route everything through there?

Our fixes branch is on 6.17-rc1. You can use it as a base if you want
to.

> 
> Alternatively, frontload the MSHV fixes (which I'll do regardless) and take those
> through hyperv and the rest through the tip tree?  That seems like an absurd
> amount of juggling though, especially if we want to get the cleanups into 6.18.
> And if none of these lands, it's MSHV that'll suffer the most, so betting it all
> on the hyperv tree doesn't seem terrible.
> 

I'm happy to do it however the community sees fit.

> > Also, is it possible to make all the mshv driver changes in a single patch?
> 
> It's certainly possible, but I'd prefer not do to that.
> 
> > It seems like it would be cleaner than refactoring it in patches 1 & 2 and
> > then deleting all the refactored code in patch 5.
> 
> Only if you don't care about backporting fixes, bisection, or maintaining code.
> 
> E.g. if checking NEED_RESCHED_LAZY somehow causes issues, it would be really nice
> for that to bisect to exactly that patch, not a patch that also switches to a
> completely different set of APIs.
> 
> And if someone is wants the fixes in a pre-6.18 kernel, they don't need to
> backport all of the KVM and entry code changes just to get the fix.

+1 on this.

Thanks,
Wei

> 
> As for the maintenance headache, see above.

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