linux-kernel - Re: [PATCH 3/3] x86/xen: allow nesting of same lazy mode

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Message-ID: <c243936e-bda3-eb3f-1725-323772ad3a7d@oracle.com>
Date:   Fri, 15 Sep 2023 17:50:11 -0400
From:   Boris Ostrovsky <boris.ostrovsky@...cle.com>
To:     Juergen Gross <jgross@...e.com>, linux-kernel@...r.kernel.org,
        x86@...nel.org
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "H. Peter Anvin" <hpa@...or.com>, xen-devel@...ts.xenproject.org
Subject: Re: [PATCH 3/3] x86/xen: allow nesting of same lazy mode



On 9/13/23 7:38 AM, Juergen Gross wrote:
> When running as a paravirtualized guest under Xen, Linux is using
> "lazy mode" for issuing hypercalls which don't need to take immediate
> effect in order to improve performance (examples are e.g. multiple
> PTE changes).
> 
> There are two different lazy modes defined: MMU and CPU lazy mode.
> Today it is not possible to nest multiple lazy mode sections, even if
> they are of the same kind. A recent change in memory management added
> nesting of MMU lazy mode sections, resulting in a regression when
> running as Xen PV guest.
> 
> Technically there is no reason why nesting of multiple sections of the
> same kind of lazy mode shouldn't be allowed. So add support for that
> for fixing the regression.
> 
> Fixes: bcc6cc832573 ("mm: add default definition of set_ptes()")
> Signed-off-by: Juergen Gross <jgross@...e.com>

For patches 2 and 3

Reviewed-by: Boris Ostrovsky <boris.ostrovsky@...cle.com>