Re: [PATCH v2 05/20] KVM: s390: Add helper functions for fault handling

Alexander Gordeev
2025-09-18T14:42:04Z

On Wed, Sep 10, 2025 at 08:07:31PM +0200, Claudio Imbrenda wrote:
> Add some helper functions for handling multiple guest faults at the
> same time.
>
> This will be needed for VSIE, where a nested guest access also needs to
> access all the page tables that map it.
>
> Signed-off-by: Claudio Imbrenda <imbrenda@linux.ibm.com>
> ---
> arch/s390/kvm/gaccess.h | 14 ++++++++++
> arch/s390/kvm/kvm-s390.c | 44 +++++++++++++++++++++++++++++++
> arch/s390/kvm/kvm-s390.h | 56 ++++++++++++++++++++++++++++++++++++++++
> 3 files changed, 114 insertions(+)
>
> diff --git a/arch/s390/kvm/gaccess.h b/arch/s390/kvm/gaccess.h
> index 3fde45a151f2..9c82f7460821 100644
> --- a/arch/s390/kvm/gaccess.h
> +++ b/arch/s390/kvm/gaccess.h
> @@ -457,4 +457,18 @@ int kvm_s390_check_low_addr_prot_real(struct kvm_vcpu *vcpu, unsigned long gra);
> int kvm_s390_shadow_fault(struct kvm_vcpu *vcpu, struct gmap *shadow,
> unsigned long saddr, unsigned long *datptr);
>
> +static inline int __kvm_s390_faultin_read_gpa(struct kvm *kvm, struct guest_fault *f, gpa_t gaddr,
> + unsigned long *val)
> +{
> + phys_addr_t phys_addr;
> + int rc;
> +
> + rc = __kvm_s390_faultin_gfn(kvm, f, gpa_to_gfn(gaddr), false);

Why not a "typical" flow (below)?

> + if (!rc) {
> + phys_addr = PFN_PHYS(f->pfn) | offset_in_page(gaddr);
> + *val = *(unsigned long *)phys_to_virt(phys_addr);
> + }
> + return rc;

if (rc)
return rc;

phys_addr = pfn_to_phys(f->pfn) | offset_in_page(gaddr);
*val = *(unsigned long *)phys_to_virt(phys_addr);

return 0;

> +}
> +
> #endif /* __KVM_S390_GACCESS_H */
> diff --git a/arch/s390/kvm/kvm-s390.c b/arch/s390/kvm/kvm-s390.c
> index 61aa64886c36..af8a62abec48 100644
> --- a/arch/s390/kvm/kvm-s390.c
> +++ b/arch/s390/kvm/kvm-s390.c
> @@ -4858,6 +4858,50 @@ static void kvm_s390_assert_primary_as(struct kvm_vcpu *vcpu)
> current->thread.gmap_int_code, current->thread.gmap_teid.val);
> }
>
> +/**
> + * __kvm_s390_faultin_gfn() - fault in and pin a guest address
> + * @kvm: the guest
> + * @guest_fault: will be filled with information on the pin operation
> + * @gfn: guest frame
> + * @wr: if true indicates a write access
> + *
> + * Fault in and pin a guest address using absolute addressing, and without
> + * marking the page referenced.
> + *
> + * Context: Called with mm->mmap_lock in read mode.
> + *
> + * Return:
> + * * 0 in case of success,
> + * * -EFAULT if reading using the virtual address failed,
> + * * -EINTR if a signal is pending,
> + * * -EAGAIN if FOLL_NOWAIT was specified, but IO is needed
> + * * PGM_ADDRESSING if the guest address lies outside of guest memory.
> + */
> +int __kvm_s390_faultin_gfn(struct kvm *kvm, struct guest_fault *guest_fault, gfn_t gfn, bool wr)
> +{
> + struct kvm_memory_slot *slot;
> + kvm_pfn_t pfn;
> + int foll;
> +
> + foll = wr ? FOLL_WRITE : 0;
> + slot = gfn_to_memslot(kvm, gfn);
> + pfn = __kvm_faultin_pfn(slot, gfn, foll, &guest_fault->writable, &guest_fault->page);
> + if (is_noslot_pfn(pfn))
> + return PGM_ADDRESSING;
> + if (is_sigpending_pfn(pfn))
> + return -EINTR;
> + if (pfn == KVM_PFN_ERR_NEEDS_IO)
> + return -EAGAIN;
> + if (is_error_pfn(pfn))
> + return -EFAULT;
> +
> + guest_fault->pfn = pfn;
> + guest_fault->gfn = gfn;
> + guest_fault->write_attempt = wr;
> + guest_fault->valid = true;
> + return 0;
> +}
> +
> /*
> * __kvm_s390_handle_dat_fault() - handle a dat fault for the gmap of a vcpu
> * @vcpu: the vCPU whose gmap is to be fixed up
> diff --git a/arch/s390/kvm/kvm-s390.h b/arch/s390/kvm/kvm-s390.h
> index c44fe0c3a097..dabcf65f58ff 100644
> --- a/arch/s390/kvm/kvm-s390.h
> +++ b/arch/s390/kvm/kvm-s390.h
> @@ -22,6 +22,15 @@
>
> #define KVM_S390_UCONTROL_MEMSLOT (KVM_USER_MEM_SLOTS + 0)
>
> +struct guest_fault {
> + gfn_t gfn; /* Guest frame */
> + kvm_pfn_t pfn; /* Host PFN */
> + struct page *page; /* Host page */
> + bool writable; /* Mapping is writable */
> + bool write_attempt; /* Write access attempted */
> + bool valid; /* This entry contains valid data */
> +};
> +
> static inline void kvm_s390_fpu_store(struct kvm_run *run)
> {
> fpu_stfpc(&run->s.regs.fpc);
> @@ -464,12 +473,59 @@ int kvm_s390_cpus_from_pv(struct kvm *kvm, u16 *rc, u16 *rrc);
> int __kvm_s390_handle_dat_fault(struct kvm_vcpu *vcpu, gfn_t gfn, gpa_t gaddr, unsigned int flags);
> int __kvm_s390_mprotect_many(struct gmap *gmap, gpa_t gpa, u8 npages, unsigned int prot,
> unsigned long bits);
> +int __kvm_s390_faultin_gfn(struct kvm *kvm, struct guest_fault *f, gfn_t gfn, bool wr);
>
> static inline int kvm_s390_handle_dat_fault(struct kvm_vcpu *vcpu, gpa_t gaddr, unsigned int flags)
> {
> return __kvm_s390_handle_dat_fault(vcpu, gpa_to_gfn(gaddr), gaddr, flags);
> }
>
> +static inline void release_faultin_multiple(struct kvm *kvm, struct guest_fault *guest_faults,
> + int n, bool ignore)
> +{
> + int i;
> +
> + for (i = 0; i < n; i++) {
> + kvm_release_faultin_page(kvm, guest_faults[i].page, ignore,
> + guest_faults[i].write_attempt);
> + guest_faults[i].page = NULL;
> + }
> +}
> +
> +static inline bool __kvm_s390_multiple_faults_need_retry(struct kvm *kvm, unsigned long seq,
> + struct guest_fault *guest_faults, int n,
> + bool unsafe)
> +{
> + int i;
> +
> + for (i = 0; i < n; i++) {
> + if (!guest_faults[i].valid)
> + continue;
> + if ((unsafe && mmu_invalidate_retry_gfn_unsafe(kvm, seq, guest_faults[i].gfn)) ||
> + (!unsafe && mmu_invalidate_retry_gfn(kvm, seq, guest_faults[i].gfn))) {
> + release_faultin_multiple(kvm, guest_faults, n, true);
> + return true;
> + }
> + }
> + return false;
> +}
> +
> +static inline int __kvm_s390_faultin_gfn_range(struct kvm *kvm, struct guest_fault *guest_faults,
> + gfn_t start, int n_pages, bool write_attempt)
> +{
> + int i, rc = 0;
> +
> + for (i = 0; !rc && i < n_pages; i++)
> + rc = __kvm_s390_faultin_gfn(kvm, guest_faults + i, start + i, write_attempt);
> + return rc;
> +}
> +
> +#define release_faultin_array(kvm, array, ignore) \
> + release_faultin_multiple(kvm, array, ARRAY_SIZE(array), ignore)
> +
> +#define __kvm_s390_fault_array_needs_retry(kvm, seq, array, unsafe) \
> + __kvm_s390_multiple_faults_need_retry(kvm, seq, array, ARRAY_SIZE(array), unsafe)
> +
> /* implemented in diag.c */
> int kvm_s390_handle_diag(struct kvm_vcpu *vcpu);
>
> --
> 2.51.0
>