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Subject: Re: [PATCH v5 7/7] KVM: PPC: Book3S HV: rework secure mem slot
 dropping
To: bharata@linux.ibm.com, Ram Pai <linuxram@us.ibm.com>
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 <20200724030337.GC1082478@in.ibm.com>
From: Laurent Dufour <ldufour@linux.ibm.com>
Message-ID: <68508568-25dd-ab57-b56c-9ce0dd7ae54d@linux.ibm.com>
Date: Fri, 24 Jul 2020 09:43:21 +0200
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Cc: cclaudio@linux.ibm.com, kvm-ppc@vger.kernel.org,
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Le 24/07/2020 à 05:03, Bharata B Rao a écrit :
> On Thu, Jul 23, 2020 at 01:07:24PM -0700, Ram Pai wrote:
>> From: Laurent Dufour <ldufour@linux.ibm.com>
>>
>> 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 call directly instead of triggering such a mechanism. This
>> 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 held 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@us.ibm.com>
>> Cc: Bharata B Rao <bharata@linux.ibm.com>
>> Cc: Paul Mackerras <paulus@ozlabs.org>
>> Signed-off-by: Ram Pai <linuxram@us.ibm.com>
>> 	[modified the changelog description]
>> Signed-off-by: Laurent Dufour <ldufour@linux.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 c772e92..daffa6e 100644
>> --- a/arch/powerpc/kvm/book3s_hv_uvmem.c
>> +++ b/arch/powerpc/kvm/book3s_hv_uvmem.c
>> @@ -632,35 +632,55 @@ 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;
>> +
>> +	mmap_read_lock(kvm->mm);
>> +
>> +	addr = slot->userspace_addr;
>> +	end = addr + (slot->npages * PAGE_SIZE);
>>   
>> -	for (i = free->npages; i; --i, ++gfn) {
>> -		struct page *uvmem_page;
>> +	gfn = slot->base_gfn;
>> +	for (i = slot->npages; i; --i, ++gfn, addr += PAGE_SIZE) {
>> +
>> +		/* Fetch the VMA if addr is not in the latest fetched one */
>> +		if (!vma || (addr < vma->vm_start || addr >= vma->vm_end)) {
>> +			vma = find_vma_intersection(kvm->mm, addr, end);
>> +			if (!vma ||
>> +			    vma->vm_start > addr || vma->vm_end < end) {
>> +				pr_err("Can't find VMA for gfn:0x%lx\n", gfn);
>> +				break;
>> +			}
> 
> There is a potential issue with the boundary condition check here
> which I discussed with Laurent yesterday. Guess he hasn't gotten around
> to look at it yet.

Right, I'm working on that..