Re: [PATCH v3 03/15] KVM: s390: fake memslot for ucontrol VMs

["Christoph Schlameuss" <schlameuss@linux.ibm.com>]

On Fri Jan 17, 2025 at 8:09 PM CET, Claudio Imbrenda wrote:
> Create a fake memslot for ucontrol VMs. The fake memslot identity-maps
> userspace.
>
> Now memslots will always be present, and ucontrol is not a special case
> anymore.
>
> Suggested-by: Sean Christopherson <seanjc@google.com>
> Signed-off-by: Claudio Imbrenda <imbrenda@linux.ibm.com>

LGTM assuming the triggered warning about the slot_lock can be resolved in
another patch.
Tested in G1 and G2 using the ucontrol selftests.

Reviewed-by: Christoph Schlameuss <schlameuss@linux.ibm.com>
Tested-by: Christoph Schlameuss <schlameuss@linux.ibm.com>

> ---
>  Documentation/virt/kvm/api.rst   |  2 +-
>  arch/s390/include/asm/kvm_host.h |  2 ++
>  arch/s390/kvm/kvm-s390.c         | 15 ++++++++++++++-
>  arch/s390/kvm/kvm-s390.h         |  2 ++
>  4 files changed, 19 insertions(+), 2 deletions(-)
>
> diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
> index f15b61317aad..cc98115a96d7 100644
> --- a/Documentation/virt/kvm/api.rst
> +++ b/Documentation/virt/kvm/api.rst
> @@ -1419,7 +1419,7 @@ fetch) is injected in the guest.
>  S390:
>  ^^^^^
>  
> -Returns -EINVAL if the VM has the KVM_VM_S390_UCONTROL flag set.
> +Returns -EINVAL or -EEXIST if the VM has the KVM_VM_S390_UCONTROL flag set.
>  Returns -EINVAL if called on a protected VM.
>  
>  4.36 KVM_SET_TSS_ADDR
> diff --git a/arch/s390/include/asm/kvm_host.h b/arch/s390/include/asm/kvm_host.h
> index 97c7c8127543..9df37361bc64 100644
> --- a/arch/s390/include/asm/kvm_host.h
> +++ b/arch/s390/include/asm/kvm_host.h
> @@ -30,6 +30,8 @@
>  #define KVM_S390_ESCA_CPU_SLOTS 248
>  #define KVM_MAX_VCPUS 255
>  
> +#define KVM_INTERNAL_MEM_SLOTS 1
> +
>  /*
>   * These seem to be used for allocating ->chip in the routing table, which we
>   * don't use. 1 is as small as we can get to reduce the needed memory. If we
> diff --git a/arch/s390/kvm/kvm-s390.c b/arch/s390/kvm/kvm-s390.c
> index ecbdd7d41230..58cc7f7444e5 100644
> --- a/arch/s390/kvm/kvm-s390.c
> +++ b/arch/s390/kvm/kvm-s390.c
> @@ -3428,8 +3428,18 @@ int kvm_arch_init_vm(struct kvm *kvm, unsigned long type)
>  	VM_EVENT(kvm, 3, "vm created with type %lu", type);
>  
>  	if (type & KVM_VM_S390_UCONTROL) {
> +		struct kvm_userspace_memory_region2 fake_memslot = {
> +			.slot = KVM_S390_UCONTROL_MEMSLOT,
> +			.guest_phys_addr = 0,
> +			.userspace_addr = 0,
> +			.memory_size = ALIGN_DOWN(TASK_SIZE, _SEGMENT_SIZE),
> +			.flags = 0,
> +		};
> +
>  		kvm->arch.gmap = NULL;
>  		kvm->arch.mem_limit = KVM_S390_NO_MEM_LIMIT;
> +		/* one flat fake memslot covering the whole address-space */
> +		KVM_BUG_ON(kvm_set_internal_memslot(kvm, &fake_memslot), kvm);

In the current state of kvm_set_internal_memslot this does not acquire the
slot_lock and issues a warning. I did bring this up on Seans patch introducing
the method. So I assume at this point this here is fine.

>  	} else {
>  		if (sclp.hamax == U64_MAX)
>  			kvm->arch.mem_limit = TASK_SIZE_MAX;
> @@ -5854,7 +5864,7 @@ int kvm_arch_prepare_memory_region(struct kvm *kvm,
>  {
>  	gpa_t size;
>  
> -	if (kvm_is_ucontrol(kvm))
> +	if (kvm_is_ucontrol(kvm) && new->id < KVM_USER_MEM_SLOTS)
>  		return -EINVAL;
>  
>  	/* When we are protected, we should not change the memory slots */
> @@ -5906,6 +5916,9 @@ void kvm_arch_commit_memory_region(struct kvm *kvm,
>  {
>  	int rc = 0;
>  
> +	if (kvm_is_ucontrol(kvm))
> +		return;
> +
>  	switch (change) {
>  	case KVM_MR_DELETE:
>  		rc = gmap_unmap_segment(kvm->arch.gmap, old->base_gfn * PAGE_SIZE,
> diff --git a/arch/s390/kvm/kvm-s390.h b/arch/s390/kvm/kvm-s390.h
> index 597d7a71deeb..30736ac16f84 100644
> --- a/arch/s390/kvm/kvm-s390.h
> +++ b/arch/s390/kvm/kvm-s390.h
> @@ -20,6 +20,8 @@
>  #include <asm/processor.h>
>  #include <asm/sclp.h>
>  
> +#define KVM_S390_UCONTROL_MEMSLOT (KVM_USER_MEM_SLOTS + 0)
> +
>  static inline void kvm_s390_fpu_store(struct kvm_run *run)
>  {
>  	fpu_stfpc(&run->s.regs.fpc);