Re: [PATCH v1 1/2] KVM: s390: load guest access registers in MEM_OP ioctl

[Heiko Carstens <hca@linux.ibm.com>]

On Mon, Feb 12, 2024 at 11:21:30AM +0100, Heiko Carstens wrote:
> Or maybe a TIF flag with different semantics: "guest save area does not
> reflect current state - which is within registers".

Something like the below; untested of course. But I guess there must be
some arch specific vcpu flags, which can be used to achieve the same?

diff --git a/arch/s390/include/asm/thread_info.h b/arch/s390/include/asm/thread_info.h
index a674c7d25da5..b9ff8b125fb8 100644
--- a/arch/s390/include/asm/thread_info.h
+++ b/arch/s390/include/asm/thread_info.h
@@ -69,6 +69,7 @@ void arch_setup_new_exec(void);
 #define TIF_PATCH_PENDING	5	/* pending live patching update */
 #define TIF_PGSTE		6	/* New mm's will use 4K page tables */
 #define TIF_NOTIFY_SIGNAL	7	/* signal notifications exist */
+#define TIF_KVM_ACRS		8	/* access registers contain guest content */
 #define TIF_ISOLATE_BP_GUEST	9	/* Run KVM guests with isolated BP */
 #define TIF_PER_TRAP		10	/* Need to handle PER trap on exit to usermode */
 
diff --git a/arch/s390/kvm/gaccess.c b/arch/s390/kvm/gaccess.c
index 5bfcc50c1a68..b0ef242d2371 100644
--- a/arch/s390/kvm/gaccess.c
+++ b/arch/s390/kvm/gaccess.c
@@ -391,7 +391,8 @@ static int ar_translation(struct kvm_vcpu *vcpu, union asce *asce, u8 ar,
 	if (ar >= NUM_ACRS)
 		return -EINVAL;
 
-	save_access_regs(vcpu->run->s.regs.acrs);
+	if (test_thread_flag(TIF_KVM_ACRS))
+		save_access_regs(vcpu->run->s.regs.acrs);
 	alet.val = vcpu->run->s.regs.acrs[ar];
 
 	if (ar == 0 || alet.val == 0) {
diff --git a/arch/s390/kvm/kvm-s390.c b/arch/s390/kvm/kvm-s390.c
index ea63ac769889..3ee0913639d5 100644
--- a/arch/s390/kvm/kvm-s390.c
+++ b/arch/s390/kvm/kvm-s390.c
@@ -4951,6 +4951,7 @@ static void sync_regs(struct kvm_vcpu *vcpu)
 	}
 	save_access_regs(vcpu->arch.host_acrs);
 	restore_access_regs(vcpu->run->s.regs.acrs);
+	set_thread_flag(TIF_KVM_ACRS);
 	/* save host (userspace) fprs/vrs */
 	save_fpu_regs();
 	vcpu->arch.host_fpregs.fpc = current->thread.fpu.fpc;
@@ -5020,6 +5021,7 @@ static void store_regs(struct kvm_vcpu *vcpu)
 	kvm_run->s.regs.pfs = vcpu->arch.pfault_select;
 	kvm_run->s.regs.pfc = vcpu->arch.pfault_compare;
 	save_access_regs(vcpu->run->s.regs.acrs);
+	clear_thread_flag(TIF_KVM_ACRS);
 	restore_access_regs(vcpu->arch.host_acrs);
 	/* Save guest register state */
 	save_fpu_regs();