linux-kernel - Re: [PATCH RFC 0/4] KVM: MIPS: Provide arch-specific kvm_flush_remote

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Message-ID: <20200212163004.cpd33ux4zslfc3es@lantea.localdomain>
Date:   Wed, 12 Feb 2020 08:30:04 -0800
From:   Paul Burton <paulburton@...nel.org>
To:     Paolo Bonzini <pbonzini@...hat.com>,
        Mike Rapoport <rppt@...ux.ibm.com>
Cc:     Peter Xu <peterx@...hat.com>, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        linux-mips@...r.kernel.org, Vitaly Kuznetsov <vkuznets@...hat.com>
Subject: Re: [PATCH RFC 0/4] KVM: MIPS: Provide arch-specific
 kvm_flush_remote_tlbs()

Hi Paolo,

On Wed, Feb 12, 2020 at 01:25:30PM +0100, Paolo Bonzini wrote:
> MIPS folks, I see that arch/mips/kvm/mmu.c uses pud_index, so it's not
> clear to me if it's meant to only work if CONFIG_PGTABLE_LEVELS=4 or
> it's just bit rot.  Should I add a "depends on PGTABLE_LEVEL=4" to
> arch/mips/Kconfig?

I'm no expert on this bit of code, but I'm pretty sure the systems
KVM/VZ has been used on the most internally had PGTABLE_LEVEL=3.

I suspect this is actually a regression from commit 31168f033e37 ("mips:
drop __pXd_offset() macros that duplicate pXd_index() ones"). Whilst
that commit is correct that pud_index() & __pud_offset() are the same
when pud_index() is actually provided, it doesn't take into account the
__PAGETABLE_PUD_FOLDED case. There __pud_offset() was available but
would always evaluate to zero, whereas pud_index() isn't defined...

Thanks,
    Paul