linux-kernel - Re: [PATCH 0/2] KVM: x86/mmu: .change

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Message-ID: <ZPkemGED1QD7kgUo@google.com>
Date:   Wed, 6 Sep 2023 17:51:36 -0700
From:   Sean Christopherson <seanjc@...gle.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     Yan Zhao <yan.y.zhao@...el.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 0/2] KVM: x86/mmu: .change_pte() optimization in TDP MMU

On Thu, Sep 07, 2023, Paolo Bonzini wrote:
> On Tue, Sep 5, 2023 at 10:18 PM Sean Christopherson <seanjc@...gle.com> wrote:
> > Ooh, actually, maybe we could do
> >
> >         static bool <name_tbd> = !IS_ENABLED(CONFIG_KSM);
> >
> > and then cross our fingers that that doesn't regress some other funky setups.
> 
> It probably breaks gvisor-like setups that use MAP_PRIVATE mmap for
> memslots? It would instantly break CoW even if memory is never
> written.

Doh, I completely forgot about gvisor and the like.

Yan, I don't think this is worth pursuing.  My understanding is that only legacy,
relatively slow devices need DMA32.  And as Robin pointed out, swiotlb=force isn't
something that's likely deployed and certainly isn't intended for performance
sensitive environments.