linux-kernel - Re: [PATCH 4/7] x86/xen: support nested lazy

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Message-ID: <d3adc2a0-5888-411e-ac7c-9df45e3389c9-agordeev@linux.ibm.com>
Date: Fri, 5 Sep 2025 17:48:23 +0200
From: Alexander Gordeev <agordeev@...ux.ibm.com>
To: Kevin Brodsky <kevin.brodsky@....com>
Cc: linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Andreas Larsson <andreas@...sler.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Borislav Petkov <bp@...en8.de>,
        Catalin Marinas <catalin.marinas@....com>,
        Christophe Leroy <christophe.leroy@...roup.eu>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        David Hildenbrand <david@...hat.com>,
        "David S. Miller" <davem@...emloft.net>,
        "H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...hat.com>,
        Jann Horn <jannh@...gle.com>, Juergen Gross <jgross@...e.com>,
        "Liam R. Howlett" <Liam.Howlett@...cle.com>,
        Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
        Madhavan Srinivasan <maddy@...ux.ibm.com>,
        Michael Ellerman <mpe@...erman.id.au>, Michal Hocko <mhocko@...e.com>,
        Mike Rapoport <rppt@...nel.org>, Nicholas Piggin <npiggin@...il.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Ryan Roberts <ryan.roberts@....com>,
        Suren Baghdasaryan <surenb@...gle.com>,
        Thomas Gleixner <tglx@...utronix.de>, Vlastimil Babka <vbabka@...e.cz>,
        Will Deacon <will@...nel.org>, linux-arm-kernel@...ts.infradead.org,
        linuxppc-dev@...ts.ozlabs.org, sparclinux@...r.kernel.org,
        xen-devel@...ts.xenproject.org
Subject: Re: [PATCH 4/7] x86/xen: support nested lazy_mmu sections (again)

On Thu, Sep 04, 2025 at 01:57:33PM +0100, Kevin Brodsky wrote:
...
> -static void xen_enter_lazy_mmu(void)
> +static lazy_mmu_state_t xen_enter_lazy_mmu(void)
>  {
> +	if (this_cpu_read(xen_lazy_mode) == XEN_LAZY_MMU)
> +		return LAZY_MMU_NESTED;
> +
>  	enter_lazy(XEN_LAZY_MMU);
> +	return LAZY_MMU_DEFAULT;
>  }
>  
>  static void xen_flush_lazy_mmu(void)
> @@ -2167,11 +2171,12 @@ static void __init xen_post_allocator_init(void)
>  	pv_ops.mmu.write_cr3 = &xen_write_cr3;
>  }
>  
> -static void xen_leave_lazy_mmu(void)
> +static void xen_leave_lazy_mmu(lazy_mmu_state_t state)
>  {
>  	preempt_disable();
>  	xen_mc_flush();
> -	leave_lazy(XEN_LAZY_MMU);
> +	if (state != LAZY_MMU_NESTED)
> +		leave_lazy(XEN_LAZY_MMU);

Based on xen_enter_lazy_mmu(), whether this condition needs to be
executed with the preemption disabled?

Or may be this_cpu_read(xen_lazy_mode) + enter_lazy(XEN_LAZY_MMU)
should be executed with the preemption disabled?

>  	preempt_enable();
>  }

Thanks!