linux-kernel - Re: [PATCH v2 2/2] KVM: PPC: Book3S HV: rework secure mem slot dropping

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Message-ID: <20200721213736.GG7339@oc0525413822.ibm.com>
Date:   Tue, 21 Jul 2020 14:37:36 -0700
From:   Ram Pai <linuxram@...ibm.com>
To:     Laurent Dufour <ldufour@...ux.ibm.com>
Cc:     linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org,
        kvm-ppc@...r.kernel.org, mpe@...erman.id.au, paulus@...ba.org,
        sukadev@...ux.ibm.com, bauerman@...ux.ibm.com,
        bharata@...ux.ibm.com, Paul Mackerras <paulus@...abs.org>
Subject: Re: [PATCH v2 2/2] KVM: PPC: Book3S HV: rework secure mem slot
 dropping

On Tue, Jul 21, 2020 at 12:42:02PM +0200, Laurent Dufour wrote:
> When a secure memslot is dropped, all the pages backed in the secure device
> (aka really backed by secure memory by the Ultravisor) should be paged out
> to a normal page. Previously, this was achieved by triggering the page
> fault mechanism which is calling kvmppc_svm_page_out() on each pages.
> 
> This can't work when hot unplugging a memory slot because the memory slot
> is flagged as invalid and gfn_to_pfn() is then not trying to access the
> page, so the page fault mechanism is not triggered.
> 
> Since the final goal is to make a call to kvmppc_svm_page_out() it seems
> simpler to directly calling it instead of triggering such a mechanism. This
            ^^ call directly instead of triggering..

> way kvmppc_uvmem_drop_pages() can be called even when hot unplugging a
> memslot.
> 
> Since kvmppc_uvmem_drop_pages() is already holding kvm->arch.uvmem_lock,
> the call to __kvmppc_svm_page_out() is made.
> As __kvmppc_svm_page_out needs the vma pointer to migrate the pages, the
> VMA is fetched in a lazy way, to not trigger find_vma() all the time. In
> addition, the mmap_sem is help in read mode during that time, not in write
		          ^^ held

> mode since the virual memory layout is not impacted, and
> kvm->arch.uvmem_lock prevents concurrent operation on the secure device.
> 
> Cc: Ram Pai <linuxram@...ibm.com>

Reviewed-by: Ram Pai <linuxram@...ibm.com>

RP