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

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Message-ID: <7eb0cde4-9436-9719-dd13-caf4ab5083a2@redhat.com>
Date:   Mon, 7 Jan 2019 17:26:35 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     lantianyu1986@...il.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, rkrcmar@...hat.com,
        tglx@...utronix.de, mingo@...hat.com, bp@...en8.de, hpa@...or.com,
        x86@...nel.org, linux-arm-kernel@...ts.infradead.org,
        kvmarm@...ts.cs.columbia.edu, linux-kernel@...r.kernel.org,
        linux-mips@...r.kernel.org, kvm-ppc@...r.kernel.org,
        linuxppc-dev@...ts.ozlabs.org, 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 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.

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).

Paolo

> +		flush = false;
> +	}
> +