Re: [PATCH RFC 3/5] KVM: s390: Shadow VSIE SCA in guest-1

[Janosch Frank <frankja@linux.ibm.com>]

On 3/18/25 7:59 PM, Christoph Schlameuss wrote:
> Introduce a new shadow_sca function into kvm_s390_handle_vsie.
> kvm_s390_handle_vsie is called within guest-1 when guest-2 initiates the
> VSIE.
> 
> shadow_sca and unshadow_sca create and manage ssca_block structs in
> guest-1 memory. References to the created ssca_blocks are kept within an
> array and limited to the number of cpus. This ensures each VSIE in
> execution can have its own SCA. Having the amount of shadowed SCAs
> configurable above this is left to another patch.
> 
> SCAOL/H in the VSIE control block are overwritten with references to the
> shadow SCA. The original SCA pointer is saved in the vsie_page and
> restored on VSIE exit. This limits the amount of change in the
> preexisting VSIE pin and shadow functions.
> 
> The shadow SCA contains the addresses of the original guest-3 SCA as
> well as the original VSIE control blocks. With these addresses the
> machine can directly monitor the intervention bits within the original
> SCA entries.
> 
> The ssca_blocks are also kept within a radix tree to reuse already
> existing ssca_blocks efficiently. While the radix tree and array with
> references to the ssca_blocks are held in kvm_s390_vsie.
> The use of the ssca_blocks is tracked using an ref_count on the block
> itself.
> 
> No strict mapping between the guest-1 vcpu and guest-3 vcpu is enforced.
> Instead each VSIE entry updates the shadow SCA creating a valid mapping
> for all cpus currently in VSIE.
> 
> Signed-off-by: Christoph Schlameuss <schlameuss@linux.ibm.com>
> ---
>   arch/s390/include/asm/kvm_host.h |  22 +++-
>   arch/s390/kvm/vsie.c             | 264 ++++++++++++++++++++++++++++++++++++++-
>   2 files changed, 281 insertions(+), 5 deletions(-)
> 
> diff --git a/arch/s390/include/asm/kvm_host.h b/arch/s390/include/asm/kvm_host.h
> index 0aca5fa01f3d772c3b3dd62a22134c0d4cb9dc22..4ab196caa9e79e4c4d295d23fed65e1a142e6ab1 100644
> --- a/arch/s390/include/asm/kvm_host.h
> +++ b/arch/s390/include/asm/kvm_host.h
> @@ -29,6 +29,7 @@
>   #define KVM_S390_BSCA_CPU_SLOTS 64
>   #define KVM_S390_ESCA_CPU_SLOTS 248
>   #define KVM_MAX_VCPUS 255
> +#define KVM_S390_MAX_VCPUS 256

#define KVM_S390_SSCA_CPU_SLOTS 256

Yes, I'm aware, that ESCA and MAX_VCPUS are pretty confusing.
I'm searching for solutions but they might take a while.

>   
>   #define KVM_INTERNAL_MEM_SLOTS 1
>   
> @@ -137,13 +138,23 @@ struct esca_block {
>   
>   /*
>    * The shadow sca / ssca needs to cover both bsca and esca depending on what the
> - * guest uses so we use KVM_S390_ESCA_CPU_SLOTS.
> + * guest uses so we allocate space for 256 entries that are defined in the
> + * architecture.
>    * The header part of the struct must not cross page boundaries.
>    */
>   struct ssca_block {
>   	__u64	osca;
>   	__u64	reserved08[7];
> -	struct ssca_entry cpu[KVM_S390_ESCA_CPU_SLOTS];
> +	struct ssca_entry cpu[KVM_S390_MAX_VCPUS];

This should have been resolved in the previous patch, no?

> +};
> +
> +/*
> + * Store the vsie ssca block and accompanied management data.
> + */
> +struct ssca_vsie {
> +	struct ssca_block ssca;			/* 0x0000 */
> +	__u8	reserved[0x2200 - 0x2040];	/* 0x2040 */
> +	atomic_t ref_count;			/* 0x2200 */
>   };
>   

[...]

>   void kvm_s390_vsie_gmap_notifier(struct gmap *gmap, unsigned long start,
>   				 unsigned long end)
>   {
> @@ -699,6 +932,9 @@ static void unpin_blocks(struct kvm_vcpu *vcpu, struct vsie_page *vsie_page)
>   
>   	hpa = (u64) scb_s->scaoh << 32 | scb_s->scaol;
>   	if (hpa) {
> +		/* with vsie_sigpif scaoh/l was pointing to g1 ssca_block but
> +		 * should have been reset in unshadow_sca()
> +		 */

There shouldn't be text in the first or last line of multi-line comments.

>   		unpin_guest_page(vcpu->kvm, vsie_page->sca_gpa, hpa);
>   		vsie_page->sca_gpa = 0;
>   		scb_s->scaol = 0;
> @@ -775,6 +1011,9 @@ static int pin_blocks(struct kvm_vcpu *vcpu, struct vsie_page *vsie_page)
>   		if (rc)
>   			goto unpin;
>   		vsie_page->sca_gpa = gpa;
> +		/* with vsie_sigpif scaoh and scaol will be overwritten
> +		 * in shadow_sca to point to g1 ssca_block instead
> +		 */

Same

>   		scb_s->scaoh = (u32)((u64)hpa >> 32);
>   		scb_s->scaol = (u32)(u64)hpa;
>   	}
> @@ -1490,12 +1729,17 @@ int kvm_s390_handle_vsie(struct kvm_vcpu *vcpu)
>   		goto out_unpin_scb;
>   	rc = pin_blocks(vcpu, vsie_page);
>   	if (rc)
> -		goto out_unshadow;
> +		goto out_unshadow_scb;
> +	rc = shadow_sca(vcpu, vsie_page);
> +	if (rc)
> +		goto out_unpin_blocks;
>   	register_shadow_scb(vcpu, vsie_page);
>   	rc = vsie_run(vcpu, vsie_page);
>   	unregister_shadow_scb(vcpu);

For personal preference I'd like to have a \n here to visually separate 
the cleanup from the rest of the code.

> +	unshadow_sca(vcpu, vsie_page);
> +out_unpin_blocks:
>   	unpin_blocks(vcpu, vsie_page);
> -out_unshadow:
> +out_unshadow_scb:
>   	unshadow_scb(vcpu, vsie_page);
>   out_unpin_scb:
>   	unpin_scb(vcpu, vsie_page, scb_addr);
> @@ -1510,12 +1754,15 @@ void kvm_s390_vsie_init(struct kvm *kvm)
>   {
>   	mutex_init(&kvm->arch.vsie.mutex);
>   	INIT_RADIX_TREE(&kvm->arch.vsie.addr_to_page, GFP_KERNEL_ACCOUNT);
> +	init_rwsem(&kvm->arch.vsie.ssca_lock);
> +	INIT_RADIX_TREE(&kvm->arch.vsie.osca_to_ssca, GFP_KERNEL_ACCOUNT);
>   }
>   
>   /* Destroy the vsie data structures. To be called when a vm is destroyed. */
>   void kvm_s390_vsie_destroy(struct kvm *kvm)
>   {
>   	struct vsie_page *vsie_page;
> +	struct ssca_vsie *ssca;
>   	int i;
>   
>   	mutex_lock(&kvm->arch.vsie.mutex);
> @@ -1531,6 +1778,17 @@ void kvm_s390_vsie_destroy(struct kvm *kvm)
>   	}
>   	kvm->arch.vsie.page_count = 0;
>   	mutex_unlock(&kvm->arch.vsie.mutex);
> +
> +	down_write(&kvm->arch.vsie.ssca_lock);
> +	for (i = 0; i < kvm->arch.vsie.ssca_count; i++) {
> +		ssca = kvm->arch.vsie.sscas[i];
> +		kvm->arch.vsie.sscas[i] = NULL;
> +		radix_tree_delete(&kvm->arch.vsie.osca_to_ssca,
> +				  (u64)phys_to_virt(ssca->ssca.osca));
> +		free_pages((unsigned long)ssca, SSCA_PAGEORDER);
> +	}
> +	kvm->arch.vsie.ssca_count = 0;
> +	up_write(&kvm->arch.vsie.ssca_lock);
>   }
>   
>   void kvm_s390_vsie_kick(struct kvm_vcpu *vcpu)
>