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Re: [PATCH 2/2] target/s390x: kvm: Honor storage keys during emulation

From: Thomas Huth
Subject: Re: [PATCH 2/2] target/s390x: kvm: Honor storage keys during emulation
Date: Wed, 25 May 2022 11:00:43 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101 Thunderbird/91.9.0

On 24/05/2022 13.52, Janis Schoetterl-Glausch wrote:
On 5/24/22 13:21, Thomas Huth wrote:
On 24/05/2022 13.10, Christian Borntraeger wrote:

Am 24.05.22 um 12:43 schrieb Thomas Huth:
On 19/05/2022 15.53, Janis Schoetterl-Glausch wrote:
On 5/19/22 12:05, Thomas Huth wrote:
On 06/05/2022 17.39, Janis Schoetterl-Glausch wrote:
Storage key controlled protection is currently not honored when
emulating instructions.
If available, enable key protection for the MEM_OP ioctl, thereby
enabling it for the s390_cpu_virt_mem_* functions, when using kvm.
As a result, the emulation of the following instructions honors storage

         The Synch I/O CLP command would need special handling in order
         to support storage keys, but is currently not supported.
      Performing commands asynchronously would require special
      handling, but commands are currently always synchronous.
      Must (and does) not change channel if terminated due to
      Suppressed on protection, works because fetching instruction.
      Suppressed on protection, works because fetching instruction.
      Suppressed on protection, this works because no partial store is
      possible, because the operand cannot span multiple pages.

Signed-off-by: Janis Schoetterl-Glausch <scgl@linux.ibm.com>
    target/s390x/kvm/kvm.c | 9 +++++++++
    1 file changed, 9 insertions(+)

diff --git a/target/s390x/kvm/kvm.c b/target/s390x/kvm/kvm.c
index 53098bf541..7bd8db0e7b 100644
--- a/target/s390x/kvm/kvm.c
+++ b/target/s390x/kvm/kvm.c
@@ -151,12 +151,15 @@ const KVMCapabilityInfo kvm_arch_required_capabilities[] 
= {
    static int cap_sync_regs;
    static int cap_async_pf;
    static int cap_mem_op;
+static int cap_mem_op_extension;
    static int cap_s390_irq;
    static int cap_ri;
    static int cap_hpage_1m;
    static int cap_vcpu_resets;
    static int cap_protected;
    +static bool mem_op_storage_key_support;
    static int active_cmma;
      static int kvm_s390_query_mem_limit(uint64_t *memory_limit)
@@ -354,6 +357,8 @@ int kvm_arch_init(MachineState *ms, KVMState *s)
        cap_sync_regs = kvm_check_extension(s, KVM_CAP_SYNC_REGS);
        cap_async_pf = kvm_check_extension(s, KVM_CAP_ASYNC_PF);
        cap_mem_op = kvm_check_extension(s, KVM_CAP_S390_MEM_OP);
+    cap_mem_op_extension = kvm_check_extension(s, 
+    mem_op_storage_key_support = cap_mem_op_extension > 0;

Ah, so KVM_CAP_S390_MEM_OP_EXTENSION is a "version number", not a boolean flag? 
... ok, now I've finally understood that ... ;-)

Yeah, potentially having a bunch of memop capabilities didn't seem nice to me.
We can remove extensions if, when introducing an extension, we define that 
version x supports functionality y, z...,
but for the storage keys I've written in api.rst that it's supported if the cap 
> 0.
So we'd need a new cap if we want to get rid of the skey extension and still 
support some other extension,
but that doesn't seem particularly likely.

Oh well, never say that ... we've seen it in the past, that sometimes we want 
to get rid of features again, and if they don't have a separate feature flag 
bit somewhere, it's getting very ugly to disable them again.

So since we don't have merged this patch yet, and thus we don't have a public 
userspace program using this interface yet, this is our last chance to redefine 
this interface before we might regret it later.

I'm in strong favor of treating the KVM_CAP_S390_MEM_OP_EXTENSION as a flag 
field instead of a version number. What do others think? Christian? Halil?

Its too late for that. This is part of 5.18.

Is it? We don't have to change the source code of the kernel,
it's just about rewording what we have in api.rst documentation
(which should be OK as long as there is no userspace program
using this yet), e.g.:

api.rst says about KVM_CHECK_EXTENSION:
:Returns: 0 if unsupported; 1 (or some other positive integer) if supported

but if we can return a negative value, we can define flags for possible future 
and flip the sign bit if we want to get rid of the storage key extension.

A bit ugly, but doesn't require any changes now.

Oh well, I hope we'll never end up in that situation ...
I guess it will likely be better to drop the MEM_OP_EXTENSION capability in that case and come up with something new instead.

Anyway, since I'm apparently the only one with my opinion, and since it's very unlikely that we want to get rid of these extensions in the future again, and we still have the big hammer of removing MEM_OP_EXTENSION completely, I won't insist on a rework here.

Queued to s390x-next now:



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