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Subject: Re: [PATCH v3 1/2] s390/kvm: extend kvm_s390_shadow_fault to return
 entry pointer
To:     Claudio Imbrenda <imbrenda@linux.ibm.com>
Cc:     linux-kernel@vger.kernel.org, borntraeger@de.ibm.com,
        frankja@linux.ibm.com, cohuck@redhat.com, kvm@vger.kernel.org,
        linux-s390@vger.kernel.org, stable@vger.kernel.org
References: <20210209154302.1033165-1-imbrenda@linux.ibm.com>
 <20210209154302.1033165-2-imbrenda@linux.ibm.com>
 <2a65f089-1728-7bc7-a2a8-a2c089a01cec@redhat.com>
 <20210211135756.249b535b@ibm-vm>
From:   David Hildenbrand <david@redhat.com>
Organization: Red Hat GmbH
Message-ID: <1fb901ef-7c42-7753-fe78-0251ca4715d3@redhat.com>
Date:   Thu, 11 Feb 2021 14:15:47 +0100
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On 11.02.21 13:57, Claudio Imbrenda wrote:
> On Thu, 11 Feb 2021 10:18:56 +0100
> David Hildenbrand <david@redhat.com> wrote:
> 
>> On 09.02.21 16:43, Claudio Imbrenda wrote:
>>> Extend kvm_s390_shadow_fault to return the pointer to the valid leaf
>>> DAT table entry, or to the invalid entry.
>>>
>>> Also return some flags in the lower bits of the address:
>>> DAT_PROT: indicates that DAT protection applies because of the
>>>             protection bit in the segment (or, if EDAT, region)
>>> tables NOT_PTE: indicates that the address of the DAT table entry
>>> returned does not refer to a PTE, but to a segment or region table.
>>>    
>>
>> I've been thinking about one possible issue, but I think it's not
>> actually an issue. Just sharing so others can verify:
>>
>> In case our guest uses huge pages / gigantic pages / ASCE R, we
>> create fake page tables (GMAP_SHADOW_FAKE_TABLE).
>>
>> So, it could be that kvm_s390_shadow_fault()->gmap_shadow_pgt_lookup()
>> succeeds, however, we have a fake PTE in our hands. We lost the
>> actual guest STE/RTE address. (I think it could be recovered somehow
>> via page->index, thought)
>>
>> But I guess, if there is a fake PTE, then there is not acutally
>> something that could go wrong in gmap_shadow_page() anymore that could
>> lead us in responding something wrong to the guest. We can only really
>> fail with -EINVAL, -ENOMEM or -EFAULT.
> 
> this was also my reasoning
> 
>> So if the guest changed anything in the meantime (e.g., zap a
>> segment), we would have unshadowed the whole fake page table
>> hierarchy and would simply retry.
>>
>>> Signed-off-by: Claudio Imbrenda <imbrenda@linux.ibm.com>
>>> Cc: stable@vger.kernel.org
>>> ---
>>>    arch/s390/kvm/gaccess.c | 30 +++++++++++++++++++++++++-----
>>>    arch/s390/kvm/gaccess.h |  5 ++++-
>>>    arch/s390/kvm/vsie.c    |  8 ++++----
>>>    3 files changed, 33 insertions(+), 10 deletions(-)
>>>
>>> diff --git a/arch/s390/kvm/gaccess.c b/arch/s390/kvm/gaccess.c
>>> index 6d6b57059493..e0ab83f051d2 100644
>>> --- a/arch/s390/kvm/gaccess.c
>>> +++ b/arch/s390/kvm/gaccess.c
>>> @@ -976,7 +976,9 @@ int kvm_s390_check_low_addr_prot_real(struct
>>> kvm_vcpu *vcpu, unsigned long gra)
>>>     * kvm_s390_shadow_tables - walk the guest page table and create
>>> shadow tables
>>>     * @sg: pointer to the shadow guest address space structure
>>>     * @saddr: faulting address in the shadow gmap
>>> - * @pgt: pointer to the page table address result
>>> + * @pgt: pointer to the beginning of the page table for the given
>>> address if
>>> + *       successful (return value 0), or to the first invalid DAT
>>> entry in
>>> + *       case of exceptions (return value > 0)
>>>     * @fake: pgt references contiguous guest memory block, not a
>>> pgtable */
>>>    static int kvm_s390_shadow_tables(struct gmap *sg, unsigned long
>>> saddr, @@ -1034,6 +1036,7 @@ static int
>>> kvm_s390_shadow_tables(struct gmap *sg, unsigned long saddr,
>>> rfte.val = ptr; goto shadow_r2t;
>>>    		}
>>> +		*pgt = ptr + vaddr.rfx * 8;
>>>    		rc = gmap_read_table(parent, ptr + vaddr.rfx * 8,
>>> &rfte.val);
>>
>> Using
>>
>> gmap_read_table(parent, *pgt, &rfte.val);
>>
>> or similar with a local variable might then be even clearer. But no
>> strong opinion.
> 
> that's also something I had thought about, in the end this minimizes
> the number of lines / variables while still being readable
> 
>>>    		if (rc)
>>>    			return rc;
>>> @@ -1060,6 +1063,7 @@ static int kvm_s390_shadow_tables(struct gmap
>>> *sg, unsigned long saddr, rste.val = ptr;
>>>    			goto shadow_r3t;
>>>    		}
>>> +		*pgt = ptr + vaddr.rsx * 8;
>>>    		rc = gmap_read_table(parent, ptr + vaddr.rsx * 8,
>>> &rste.val); if (rc)
>>>    			return rc;
>>> @@ -1087,6 +1091,7 @@ static int kvm_s390_shadow_tables(struct gmap
>>> *sg, unsigned long saddr, rtte.val = ptr;
>>>    			goto shadow_sgt;
>>>    		}
>>> +		*pgt = ptr + vaddr.rtx * 8;
>>>    		rc = gmap_read_table(parent, ptr + vaddr.rtx * 8,
>>> &rtte.val); if (rc)
>>>    			return rc;
>>> @@ -1123,6 +1128,7 @@ static int kvm_s390_shadow_tables(struct gmap
>>> *sg, unsigned long saddr, ste.val = ptr;
>>>    			goto shadow_pgt;
>>>    		}
>>> +		*pgt = ptr + vaddr.sx * 8;
>>>    		rc = gmap_read_table(parent, ptr + vaddr.sx * 8,
>>> &ste.val); if (rc)
>>>    			return rc;
>>> @@ -1157,6 +1163,8 @@ static int kvm_s390_shadow_tables(struct gmap
>>> *sg, unsigned long saddr,
>>>     * @vcpu: virtual cpu
>>>     * @sg: pointer to the shadow guest address space structure
>>>     * @saddr: faulting address in the shadow gmap
>>> + * @datptr: will contain the address of the faulting DAT table
>>> entry, or of
>>> + *          the valid leaf, plus some flags
>>>     *
>>>     * Returns: - 0 if the shadow fault was successfully resolved
>>>     *	    - > 0 (pgm exception code) on exceptions while
>>> faulting @@ -1165,11 +1173,11 @@ static int
>>> kvm_s390_shadow_tables(struct gmap *sg, unsigned long saddr,
>>>     *	    - -ENOMEM if out of memory
>>>     */
>>>    int kvm_s390_shadow_fault(struct kvm_vcpu *vcpu, struct gmap *sg,
>>> -			  unsigned long saddr)
>>> +			  unsigned long saddr, unsigned long
>>> *datptr) {
>>>    	union vaddress vaddr;
>>>    	union page_table_entry pte;
>>> -	unsigned long pgt;
>>> +	unsigned long pgt = 0;
>>>    	int dat_protection, fake;
>>>    	int rc;
>>>    
>>> @@ -1191,8 +1199,20 @@ int kvm_s390_shadow_fault(struct kvm_vcpu
>>> *vcpu, struct gmap *sg, pte.val = pgt + vaddr.px * PAGE_SIZE;
>>>    		goto shadow_page;
>>>    	}
>>> -	if (!rc)
>>> -		rc = gmap_read_table(sg->parent, pgt + vaddr.px *
>>> 8, &pte.val); +
>>> +	switch (rc) {
>>> +	case PGM_SEGMENT_TRANSLATION:
>>> +	case PGM_REGION_THIRD_TRANS:
>>> +	case PGM_REGION_SECOND_TRANS:
>>> +	case PGM_REGION_FIRST_TRANS:
>>> +		pgt |= NOT_PTE;
>>> +		break;
>>> +	case 0:
>>> +		pgt += vaddr.px * 8;
>>> +		rc = gmap_read_table(sg->parent, pgt, &pte.val);
>>> +	}
>>> +	if (*datptr)
>>> +		*datptr = pgt | dat_protection * DAT_PROT;
>>>    	if (!rc && pte.i)
>>>    		rc = PGM_PAGE_TRANSLATION;
>>>    	if (!rc && pte.z)
>>> diff --git a/arch/s390/kvm/gaccess.h b/arch/s390/kvm/gaccess.h
>>> index f4c51756c462..fec26bbb17ba 100644
>>> --- a/arch/s390/kvm/gaccess.h
>>> +++ b/arch/s390/kvm/gaccess.h
>>> @@ -359,7 +359,10 @@ void ipte_unlock(struct kvm_vcpu *vcpu);
>>>    int ipte_lock_held(struct kvm_vcpu *vcpu);
>>>    int kvm_s390_check_low_addr_prot_real(struct kvm_vcpu *vcpu,
>>> unsigned long gra);
>>> +#define DAT_PROT 2
>>> +#define NOT_PTE 4
>>
>> What if our guest is using ASCE.R ?
> 
> then we don't care.
> 
> if the guest is using ASCE.R, then shadowing will always succeed, and
> the VSIE MVPG handler will retry right away.
> 
> if you are worried about the the lowest order bit, it can only be set
> if a specific feature is enabled in the host, and KVM doesn't use /
> support it, so the guest can't use it for its guest.

Got it, thanks! :)

Reviewed-by: David Hildenbrand <david@redhat.com>


-- 
Thanks,

David / dhildenb