linux-kernel - Re: [PATCH v7 06/12] KVM: guest_memfd: add module param for disabling TLB flushing

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Message-ID: <82bff1c4-987f-46cb-833c-bd99eaa46e7a@intel.com>
Date: Thu, 25 Sep 2025 12:59:13 -0700
From: Dave Hansen <dave.hansen@...el.com>
To: David Hildenbrand <david@...hat.com>, "Roy, Patrick" <roypat@...zon.co.uk>
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Subject: Re: [PATCH v7 06/12] KVM: guest_memfd: add module param for disabling
 TLB flushing

On 9/25/25 12:20, David Hildenbrand wrote:
> On 25.09.25 20:27, Dave Hansen wrote:
>> On 9/24/25 08:22, Roy, Patrick wrote:
>>> Add an option to not perform TLB flushes after direct map manipulations.
>>
>> I'd really prefer this be left out for now. It's a massive can of worms.
>> Let's agree on something that works and has well-defined behavior before
>> we go breaking it on purpose.
> 
> May I ask what the big concern here is?

It's not a _big_ concern. I just think we want to start on something
like this as simple, secure, and deterministic as possible.

Let's say that with all the unmaps that load_unaligned_zeropad() faults
start to bite us. It'll take longer to find them if the TLB isn't flushed.

Basically, it'll make the bad things happen sooner rather than later.