From: Patrick Roy <patrick.roy@linux.dev>
To: David Hildenbrand <david@redhat.com>,
Dave Hansen <dave.hansen@intel.com>,
"Roy, Patrick" <roypat@amazon.co.uk>
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Subject: Re: [PATCH v7 06/12] KVM: guest_memfd: add module param for disabling TLB flushing
Date: Fri, 26 Sep 2025 10:46:15 +0100 [thread overview]
Message-ID: <c88514c3-e15f-4853-8acf-15e7b4b979f4@linux.dev> (raw)
In-Reply-To: <c79173d8-6f18-40fa-9621-e691990501e4@redhat.com>
On Thu, 2025-09-25 at 21:13 +0100, David Hildenbrand wrote:
> On 25.09.25 21:59, Dave Hansen wrote:
>> 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.
>
> Oh, I read "can of worms" and thought there is something seriously problematic :)
>
>> I just think we want to start on something
>> like this as simple, secure, and deterministic as possible.
>
> Yes, I agree. And it should be the default. Less secure would have to be opt-in and documented thoroughly.
Yes, I am definitely happy to have the 100% secure behavior be the
default, and the skipping of TLB flushes be an opt-in, with thorough
documentation!
But I would like to include the "skip tlb flushes" option as part of
this patch series straight away, because as I was alluding to in the
commit message, with TLB flushes this is not usable for Firecracker for
performance reasons :(
>>
>> 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.
>
> Agreed.
>
Best,
Patrick
next prev parent reply other threads:[~2025-09-26 9:46 UTC|newest]
Thread overview: 56+ messages / expand[flat|nested] mbox.gz Atom feed top
2025-09-24 15:10 [PATCH v7 00/12] Direct Map Removal Support for guest_memfd Patrick Roy
2025-09-24 15:10 ` [PATCH v7 01/12] arch: export set_direct_map_valid_noflush to KVM module Patrick Roy
2025-09-24 15:10 ` Patrick Roy
2025-09-24 15:10 ` [PATCH v7 02/12] x86/tlb: export flush_tlb_kernel_range " Patrick Roy
2025-09-24 15:10 ` [PATCH v7 03/12] mm: introduce AS_NO_DIRECT_MAP Patrick Roy
2025-09-24 15:22 ` [PATCH v7 04/12] KVM: guest_memfd: Add stub for kvm_arch_gmem_invalidate Roy, Patrick
2025-09-24 15:22 ` [PATCH v7 05/12] KVM: guest_memfd: Add flag to remove from direct map Roy, Patrick
2025-09-25 11:00 ` David Hildenbrand
2025-09-25 15:52 ` Roy, Patrick
2025-09-25 19:28 ` David Hildenbrand
2025-09-26 14:49 ` Patrick Roy
2025-10-31 17:30 ` Brendan Jackman
2025-11-01 9:39 ` Mike Rapoport
2025-11-03 10:35 ` Brendan Jackman
2025-11-03 10:50 ` Mike Rapoport
2025-11-04 11:08 ` Brendan Jackman
2025-11-10 12:34 ` Mike Rapoport
2025-11-03 7:57 ` Aneesh Kumar K.V
2025-12-05 17:23 ` Nikita Kalyazin
2025-09-24 15:22 ` [PATCH v7 06/12] KVM: guest_memfd: add module param for disabling TLB flushing Roy, Patrick
2025-09-25 11:02 ` David Hildenbrand
2025-09-25 15:50 ` Roy, Patrick
2025-09-25 19:32 ` David Hildenbrand
2025-09-25 18:27 ` Dave Hansen
2025-09-25 19:20 ` David Hildenbrand
2025-09-25 19:59 ` Dave Hansen
2025-09-25 20:13 ` David Hildenbrand
2025-09-26 9:46 ` Patrick Roy [this message]
2025-09-26 10:53 ` Will Deacon
2025-09-26 20:09 ` David Hildenbrand
2025-09-27 7:38 ` Patrick Roy
2025-09-29 10:20 ` David Hildenbrand
2025-10-11 14:32 ` Patrick Roy
2025-11-07 15:29 ` Ackerley Tng
2025-11-07 17:22 ` Nikita Kalyazin
2025-11-07 17:21 ` Nikita Kalyazin
2025-10-30 16:05 ` Brendan Jackman
2025-10-31 18:31 ` Brendan Jackman
2025-09-24 15:22 ` [PATCH v7 07/12] KVM: selftests: load elf via bounce buffer Roy, Patrick
2025-09-24 15:22 ` [PATCH v7 08/12] KVM: selftests: set KVM_MEM_GUEST_MEMFD in vm_mem_add() if guest_memfd != -1 Roy, Patrick
2025-09-24 15:22 ` [PATCH v7 09/12] KVM: selftests: Add guest_memfd based vm_mem_backing_src_types Roy, Patrick
2025-09-24 15:22 ` [PATCH v7 10/12] KVM: selftests: cover GUEST_MEMFD_FLAG_NO_DIRECT_MAP in existing selftests Roy, Patrick
2025-09-24 15:22 ` [PATCH v7 11/12] KVM: selftests: stuff vm_mem_backing_src_type into vm_shape Roy, Patrick
2025-09-24 15:22 ` [PATCH v7 12/12] KVM: selftests: Test guest execution from direct map removed gmem Roy, Patrick
2025-10-30 17:18 ` Brendan Jackman
2025-12-05 17:24 ` Nikita Kalyazin
2025-09-25 10:26 ` [PATCH v7 04/12] KVM: guest_memfd: Add stub for kvm_arch_gmem_invalidate David Hildenbrand
2025-09-25 10:25 ` [PATCH v7 03/12] mm: introduce AS_NO_DIRECT_MAP David Hildenbrand
2025-09-24 15:29 ` [PATCH v7 00/12] Direct Map Removal Support for guest_memfd Roy, Patrick
2025-09-24 15:38 ` David Hildenbrand
2025-11-07 15:54 ` Brendan Jackman
2025-11-07 17:23 ` Nikita Kalyazin
2025-11-07 18:04 ` Brendan Jackman
2025-11-07 18:11 ` Nikita Kalyazin
2025-11-10 15:36 ` Brendan Jackman
2025-11-07 17:37 ` Brendan Jackman
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