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Date: Tue, 7 Apr 2026 14:27:52 -0700 (PDT)
From: Jork Loeser <jloeser@linux.microsoft.com>
To: Anirudh Rayabharam <anirudh@anirudhrb.com>
cc: linux-hyperv@vger.kernel.org, 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>, Long Li <longli@microsoft.com>, 
    Thomas Gleixner <tglx@kernel.org>, Ingo Molnar <mingo@redhat.com>, 
    Borislav Petkov <bp@alien8.de>, Dave Hansen <dave.hansen@linux.intel.com>, 
    "H . Peter Anvin" <hpa@zytor.com>, Arnd Bergmann <arnd@arndb.de>, 
    Michael Kelley <mhklinux@outlook.com>, linux-kernel@vger.kernel.org, 
    linux-arch@vger.kernel.org
Subject: Re: [PATCH v2 4/6] mshv: limit SynIC management to MSHV-owned
 resources
In-Reply-To: <20260406-ninja-civet-of-tornado-67ff54@anirudhrb>
Message-ID: <134ce833-544-24eb-883-b190a888b31c@linux.microsoft.com>
References: <20260403190613.47026-1-jloeser@linux.microsoft.com> <20260403190613.47026-5-jloeser@linux.microsoft.com> <20260406-ninja-civet-of-tornado-67ff54@anirudhrb>
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On Mon, 6 Apr 2026, Anirudh Rayabharam wrote:

> On Fri, Apr 03, 2026 at 12:06:10PM -0700, Jork Loeser wrote:
>> The SynIC is shared between VMBus and MSHV. VMBus owns the message
>> page (SIMP), event flags page (SIEFP), global enable (SCONTROL),
>> and SINT2. MSHV adds SINT0, SINT5, and the event ring page (SIRBP).
>>
>> Currently mshv_synic_init() redundantly enables SIMP, SIEFP, and
>
> The redundant enable is probably a no-op from the hypervisor side so it
> probably doesn't hurt us. The main problem is with the tear down.

It's an MSR intercept. If we can replace this by an "if()" we shave a few 
cycles.

> An alternative approach could be: check if SIMP/SIEFP/SCONTROL is
> already enabled. If so, don't enable it again. If not enabled, enable it
> and keep track of what all stuf we have enabled. Then disable all of
> them during cleanup. This approach makes less assumptions about the
> behavior of the VMBUS driver and what stuff it does or doesn't use.

It would, yes. Then again, we drag yet more state and make debugging more 
complicated / less clear to reason what happens dynamically. I had been 
debating this briefly myself, and ultimately decided against it for that 
very reason.

Best,
Jork