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

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Message-ID: <4a3caeaf-cd0c-fcd7-0a97-f367a5f78dac@linux.ibm.com>
Date:   Thu, 23 Jul 2020 14:32:30 +0200
From:   Laurent Dufour <ldufour@...ux.ibm.com>
To:     bharata@...ux.ibm.com, linuxram@...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,
        Paul Mackerras <paulus@...abs.org>
Subject: Re: [PATCH v2 2/2] KVM: PPC: Book3S HV: rework secure mem slot
 dropping

Le 23/07/2020 à 05:36, Bharata B Rao a écrit :
> 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
>> 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
>> 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 5a4b02d3f651..ba5c7c77cc3a 100644
>> --- a/arch/powerpc/kvm/book3s_hv_uvmem.c
>> +++ b/arch/powerpc/kvm/book3s_hv_uvmem.c
>> @@ -624,35 +624,55 @@ 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;
> 
> We typically use gfn_to_hva() for that, but that won't work for a
> memslot that is already marked INVALID which is the case here.
> I think it is ok to access slot->userspace_addr here of an INVALID
> memslot, but just thought of explictly bringing this up.

Which explicitly mentioned above in the patch's description:

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.

> 
>> +	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;
>> +			}
>> +		}
> 
> In Ram's series, kvmppc_memslot_page_merge() also walks the VMAs spanning
> the memslot, but it uses a different logic for the same. Why can't these
> two cases use the same method to walk the VMAs? Is there anything subtly
> different between the two cases?

This is probably doable. At the time I wrote that patch, the 
kvmppc_memslot_page_merge() was not yet introduced AFAIR.

This being said, I'd help a lot to factorize that code... I let Ram dealing with 
that ;)

Cheers,
Laurent.