linux-kernel - Re: [PATCH 9/11] KVM/MMU: Flush tlb in the kvm_mmu_write_protect_pt

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Date:   Thu, 10 Jan 2019 17:06:25 +0800
From:   Tianyu Lan <lantianyu1986@...il.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     Lan Tianyu <Tianyu.Lan@...rosoft.com>, christoffer.dall@....com,
        marc.zyngier@....com, linux@...linux.org.uk,
        catalin.marinas@....com, will.deacon@....com, jhogan@...nel.org,
        ralf@...ux-mips.org, paul.burton@...s.com, paulus@...abs.org,
        benh@...nel.crashing.org, mpe@...erman.id.au,
        Radim Krcmar <rkrcmar@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, bp@...en8.de,
        "H. Peter Anvin" <hpa@...or.com>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.cs.columbia.edu,
        "linux-kernel@...r kernel org" <linux-kernel@...r.kernel.org>,
        linux-mips@...r.kernel.org, kvm-ppc@...r.kernel.org,
        linuxppc-dev@...ts.ozlabs.org, kvm <kvm@...r.kernel.org>,
        michael.h.kelley@...rosoft.com, kys@...rosoft.com,
        vkuznets@...hat.com
Subject: Re: [PATCH 9/11] KVM/MMU: Flush tlb in the kvm_mmu_write_protect_pt_masked()

On Tue, Jan 8, 2019 at 12:26 AM Paolo Bonzini <pbonzini@...hat.com> wrote:
>
> On 04/01/19 09:54, lantianyu1986@...il.com wrote:
> >               rmap_head = __gfn_to_rmap(slot->base_gfn + gfn_offset + __ffs(mask),
> >                                         PT_PAGE_TABLE_LEVEL, slot);
> > -             __rmap_write_protect(kvm, rmap_head, false);
> > +             flush |= __rmap_write_protect(kvm, rmap_head, false);
> >
> >               /* clear the first set bit */
> >               mask &= mask - 1;
> >       }
> > +
> > +     if (flush && kvm_available_flush_tlb_with_range()) {
> > +             kvm_flush_remote_tlbs_with_address(kvm,
> > +                             slot->base_gfn + gfn_offset,
> > +                             hweight_long(mask));
>
> Mask is zero here, so this probably won't work.
>
> In addition, I suspect calling the hypercall once for every 64 pages is
> not very efficient.  Passing a flush list into
> kvm_mmu_write_protect_pt_masked, and flushing in
> kvm_arch_mmu_enable_log_dirty_pt_masked, isn't efficient either because
> kvm_arch_mmu_enable_log_dirty_pt_masked is also called once per word.
>
Yes, this is not efficient.

> I don't have any good ideas, except for moving the whole
> kvm_clear_dirty_log_protect loop into architecture-specific code (which
> is not the direction we want---architectures should share more code, not
> less).

kvm_vm_ioctl_clear_dirty_log/get_dirty_log()  is to get/clear dirty log with
memslot as unit. We may just flush tlbs of the affected memslot instead of
entire page table's when range flush is available.

>
> Paolo
>
> > +             flush = false;
> > +     }
> > +
>


--
Best regards
Tianyu Lan