Message-ID: <1b34dd89-cbbb-474e-be75-808e174c8ed9@redhat.com>
Date: Thu, 25 Sep 2025 21:32:22 +0200
From: David Hildenbrand <david@...hat.com>
To: "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 25.09.25 17:50, Roy, Patrick wrote:
> On Thu, 2025-09-25 at 12:02 +0100, David Hildenbrand wrote:
>> On 24.09.25 17:22, Roy, Patrick wrote:
>>> Add an option to not perform TLB flushes after direct map manipulations.
>>> TLB flushes result in a up to 40x elongation of page faults in
>>> guest_memfd (scaling with the number of CPU cores), or a 5x elongation
>>> of memory population, which is inacceptable when wanting to use direct
>>> map removed guest_memfd as a drop-in replacement for existing workloads.
>>>
>>> TLB flushes are not needed for functional correctness (the virt->phys
>>> mapping technically stays "correct", the kernel should simply not use it
>>> for a while), so we can skip them to keep performance in-line with
>>> "traditional" VMs.
>>>
>>> Enabling this option means that the desired protection from
>>> Spectre-style attacks is not perfect, as an attacker could try to
>>> prevent a stale TLB entry from getting evicted, keeping it alive until
>>> the page it refers to is used by the guest for some sensitive data, and
>>> then targeting it using a spectre-gadget.
>>>
>>> Cc: Will Deacon <will@...nel.org>
>>> Signed-off-by: Patrick Roy <roypat@...zon.co.uk>
>>> ---
>>>    include/linux/kvm_host.h | 1 +
>>>    virt/kvm/guest_memfd.c   | 3 ++-
>>>    virt/kvm/kvm_main.c      | 3 +++
>>>    3 files changed, 6 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
>>> index 73a15cade54a..4d2bc18860fc 100644
>>> --- a/include/linux/kvm_host.h
>>> +++ b/include/linux/kvm_host.h
>>> @@ -2298,6 +2298,7 @@ extern unsigned int halt_poll_ns;
>>>    extern unsigned int halt_poll_ns_grow;
>>>    extern unsigned int halt_poll_ns_grow_start;
>>>    extern unsigned int halt_poll_ns_shrink;
>>> +extern bool guest_memfd_tlb_flush;
>>>
>>>    struct kvm_device {
>>>        const struct kvm_device_ops *ops;
>>> diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c
>>> index b7129c4868c5..d8dd24459f0d 100644
>>> --- a/virt/kvm/guest_memfd.c
>>> +++ b/virt/kvm/guest_memfd.c
>>> @@ -63,7 +63,8 @@ static int kvm_gmem_folio_zap_direct_map(struct folio *folio)
>>>        if (!r) {
>>>                unsigned long addr = (unsigned long) folio_address(folio);
>>>                folio->private = (void *) ((u64) folio->private & KVM_GMEM_FOLIO_NO_DIRECT_MAP);
>>> -             flush_tlb_kernel_range(addr, addr + folio_size(folio));
>>> +             if (guest_memfd_tlb_flush)
>>> +                     flush_tlb_kernel_range(addr, addr + folio_size(folio));
>>>        }
>>>
>>>        return r;
>>> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
>>> index b5e702d95230..753c06ebba7f 100644
>>> --- a/virt/kvm/kvm_main.c
>>> +++ b/virt/kvm/kvm_main.c
>>> @@ -95,6 +95,9 @@ unsigned int halt_poll_ns_shrink = 2;
>>>    module_param(halt_poll_ns_shrink, uint, 0644);
>>>    EXPORT_SYMBOL_GPL(halt_poll_ns_shrink);
>>>
>>> +bool guest_memfd_tlb_flush = true;
>>> +module_param(guest_memfd_tlb_flush, bool, 0444);
>>
>> The parameter name is a bit too generic. I think you somehow have to
>> incorporate the "direct_map" aspects.
> 
> Fair :)
> 
>> Also, I wonder if this could be a capability per vm/guest_memfd?
> 
> I don't really have any opinions on how to expose this knob, but I
> thought capabilities should be additive? (e.g. we only have
> KVM_ENABLE_EXTENSION(), and then having a capability with a negative
> polarity "enable to _not_ do TLB flushes" is a bit weird in my head).

Well, you are enabling the "skip-tlbflush" feature :) So a kernel that 
knows that extension could skip tlb flushes.

So I wouldn't see this as "perform-tlbflush" but "skip-tlbflush" / 
"no-tlbflush"

> Then again, if people are fine having TLB flushes be opt-in instead of
> opt-out (Will's comment on v6 makes me believe that the opt-out itself
> might already be controversial for arm64), a capability would work.

Yeah, I think this definitely should be opt-in: opt-in to slightly less 
security in a given timeframe by performing less tlb flushes.

-- 
Cheers

David / dhildenb

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