Re: [RFC 19/37] KVM: s390: protvirt: Add new gprs location handling

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On 05.11.19 13:39, Janosch Frank wrote:
On 11/5/19 1:01 PM, Christian Borntraeger wrote:


On 04.11.19 12:25, David Hildenbrand wrote:
On 24.10.19 13:40, Janosch Frank wrote:
Guest registers for protected guests are stored at offset 0x380.

Signed-off-by: Janosch Frank <frankja@xxxxxxxxxxxxx>
---
   arch/s390/include/asm/kvm_host.h |  4 +++-
   arch/s390/kvm/kvm-s390.c         | 11 +++++++++++
   2 files changed, 14 insertions(+), 1 deletion(-)

diff --git a/arch/s390/include/asm/kvm_host.h b/arch/s390/include/asm/kvm_host.h
index 0ab309b7bf4c..5deabf9734d9 100644
--- a/arch/s390/include/asm/kvm_host.h
+++ b/arch/s390/include/asm/kvm_host.h
@@ -336,7 +336,9 @@ struct kvm_s390_itdb {
   struct sie_page {
       struct kvm_s390_sie_block sie_block;
       struct mcck_volatile_info mcck_info;    /* 0x0200 */
-    __u8 reserved218[1000];        /* 0x0218 */
+    __u8 reserved218[360];        /* 0x0218 */
+    __u64 pv_grregs[16];        /* 0x380 */
+    __u8 reserved400[512];
       struct kvm_s390_itdb itdb;    /* 0x0600 */
       __u8 reserved700[2304];        /* 0x0700 */
   };
diff --git a/arch/s390/kvm/kvm-s390.c b/arch/s390/kvm/kvm-s390.c
index 490fde080107..97d3a81e5074 100644
--- a/arch/s390/kvm/kvm-s390.c
+++ b/arch/s390/kvm/kvm-s390.c
@@ -3965,6 +3965,7 @@ static int vcpu_post_run(struct kvm_vcpu *vcpu, int exit_reason)
   static int __vcpu_run(struct kvm_vcpu *vcpu)
   {
       int rc, exit_reason;
+    struct sie_page *sie_page = (struct sie_page *)vcpu->arch.sie_block;
         /*
        * We try to hold kvm->srcu during most of vcpu_run (except when run-
@@ -3986,8 +3987,18 @@ static int __vcpu_run(struct kvm_vcpu *vcpu)
           guest_enter_irqoff();
           __disable_cpu_timer_accounting(vcpu);
           local_irq_enable();
+        if (kvm_s390_pv_is_protected(vcpu->kvm)) {
+            memcpy(sie_page->pv_grregs,
+                   vcpu->run->s.regs.gprs,
+                   sizeof(sie_page->pv_grregs));
+        }
           exit_reason = sie64a(vcpu->arch.sie_block,
                        vcpu->run->s.regs.gprs);
+        if (kvm_s390_pv_is_protected(vcpu->kvm)) {
+            memcpy(vcpu->run->s.regs.gprs,
+                   sie_page->pv_grregs,
+                   sizeof(sie_page->pv_grregs));
+        }

sie64a will load/save gprs 0-13 from to vcpu->run->s.regs.gprs.

I would have assume that this is not required for prot virt, because the HW has direct access via the sie block?

Yes, that is correct. The load/save in sie64a is not necessary for pv guests.



1. Would it make sense to have a specialized sie64a() (or a parameter, e.g., if you pass in NULL in r3), that optimizes this loading/saving? Eventually we can also optimize which host registers to save/restore then.

Having 2 kinds of sie64a seems not very nice for just saving a small number of cycles.


2. Avoid this copying here. We have to store the state to vcpu->run->s.regs.gprs when returning to user space and restore the state when coming from user space.

I like this proposal better than the first one and

It was actually an additional proposal :)

1. avoids unnecessary saving/loading/saving/restoring
2. avoids the two memcpy


Also, we access the GPRS from interception handlers, there we might use wrappers like

kvm_s390_set_gprs()
kvm_s390_get_gprs()

having register accessors might be useful anyway.
But I would like to defer that to a later point in time to keep the changes in here
minimal?

We can add a "TODO" comment in here so that we do not forget about this
for a future patch. Makes sense?

While it makes sense, I guess one could come up with a patch for 2. in less than 30 minutes ... but yeah, whatever you prefer. ;)

--

Thanks,

David / dhildenb





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