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Date: Thu, 11 Feb 2021 10:18:56 +0100
From: David Hildenbrand <david@...hat.com>
To: Claudio Imbrenda <imbrenda@...ux.ibm.com>,
linux-kernel@...r.kernel.org
Cc: borntraeger@...ibm.com, frankja@...ux.ibm.com, cohuck@...hat.com,
kvm@...r.kernel.org, linux-s390@...r.kernel.org,
stable@...r.kernel.org
Subject: Re: [PATCH v3 1/2] s390/kvm: extend kvm_s390_shadow_fault to return
entry pointer
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.
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@...ux.ibm.com>
> Cc: stable@...r.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.
> 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 ?
--
Thanks,
David / dhildenb
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