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

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Message-ID: <871rllvt4o.fsf@mpe.ellerman.id.au>
Date:   Thu, 09 Jul 2020 21:13:11 +1000
From:   Michael Ellerman <mpe@...erman.id.au>
To:     Laurent Dufour <ldufour@...ux.ibm.com>, bharata@...ux.ibm.com
Cc:     linux-kernel@...r.kernel.org, kvm-ppc@...r.kernel.org,
        linuxppc-dev@...ts.ozlabs.org, paulus@...ba.org,
        bauerman@...ux.ibm.com, sukadev@...ux.ibm.com,
        sathnaga@...ux.vnet.ibm.com, Ram Pai <linuxram@...ibm.com>,
        Paul Mackerras <paulus@...abs.org>
Subject: Re: [PATCH 2/2] KVM: PPC: Book3S HV: rework secure mem slot dropping

Laurent Dufour <ldufour@...ux.ibm.com> writes:
> Le 08/07/2020 à 13:25, Bharata B Rao a écrit :
>> On Fri, Jul 03, 2020 at 05:59:14PM +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
>>> way kvmppc_uvmem_drop_pages() can be called even when hot unplugging a
>>> memslot.
>> 
>> Yes, this appears much simpler.
>
> Thanks Bharata for reviewing this.
>
>> 
>>>
>>> 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
>>> 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>
>>> Cc: Bharata B Rao <bharata@...ux.ibm.com>
>>> Cc: Paul Mackerras <paulus@...abs.org>
>>> Signed-off-by: Laurent Dufour <ldufour@...ux.ibm.com>
>>> ---
>>>   arch/powerpc/kvm/book3s_hv_uvmem.c | 54 ++++++++++++++++++++----------
>>>   1 file changed, 37 insertions(+), 17 deletions(-)
>>>
>>> diff --git a/arch/powerpc/kvm/book3s_hv_uvmem.c b/arch/powerpc/kvm/book3s_hv_uvmem.c
>>> index 852cc9ae6a0b..479ddf16d18c 100644
>>> --- a/arch/powerpc/kvm/book3s_hv_uvmem.c
>>> +++ b/arch/powerpc/kvm/book3s_hv_uvmem.c
>>> @@ -533,35 +533,55 @@ static inline int kvmppc_svm_page_out(struct vm_area_struct *vma,
>>>    * fault on them, do fault time migration to replace the device PTEs in
>>>    * QEMU page table with normal PTEs from newly allocated pages.
>>>    */
>>> -void kvmppc_uvmem_drop_pages(const struct kvm_memory_slot *free,
>>> +void kvmppc_uvmem_drop_pages(const struct kvm_memory_slot *slot,
>>>   			     struct kvm *kvm, bool skip_page_out)
>>>   {
>>>   	int i;
>>>   	struct kvmppc_uvmem_page_pvt *pvt;
>>> -	unsigned long pfn, uvmem_pfn;
>>> -	unsigned long gfn = free->base_gfn;
>>> +	struct page *uvmem_page;
>>> +	struct vm_area_struct *vma = NULL;
>>> +	unsigned long uvmem_pfn, gfn;
>>> +	unsigned long addr, end;
>>> +
>>> +	down_read(&kvm->mm->mmap_sem);
>> 
>> You should be using mmap_read_lock(kvm->mm) with recent kernels.
>
> Absolutely, shame on me, I reviewed Michel's series about that!
>
> Paul, Michael, could you fix that when pulling this patch or should I sent a 
> whole new series?

Paul will take this series, so up to him.

cheers