Re: [PATCH v2 2/2] arch/powerpc/kvm: kvmppc_hv_entry: remove r4 argument

Michael Ellerman [off-list ref] writes:

Kautuk Consul [off-list ref] writes:

quoted

On 2023-03-15 15:48:53, Michael Ellerman wrote:

quoted

Kautuk Consul [off-list ref] writes:

quoted

kvmppc_hv_entry is called from only 2 locations within
book3s_hv_rmhandlers.S. Both of those locations set r4
as HSTATE_KVM_VCPU(r13) before calling kvmppc_hv_entry.
So, shift the r4 load instruction to kvmppc_hv_entry and
thus modify the calling convention of this function.

Signed-off-by: Kautuk Consul <redacted>
---
 arch/powerpc/kvm/book3s_hv_rmhandlers.S | 9 ++++-----
 1 file changed, 4 insertions(+), 5 deletions(-)

diff --git a/arch/powerpc/kvm/book3s_hv_rmhandlers.S b/arch/powerpc/kvm/book3s_hv_rmhandlers.S
index b81ba4ee0521..da9a15db12fe 100644
--- a/arch/powerpc/kvm/book3s_hv_rmhandlers.S
+++ b/arch/powerpc/kvm/book3s_hv_rmhandlers.S

@@ -85,7 +85,7 @@ _GLOBAL_TOC(kvmppc_hv_entry_trampoline)
 	RFI_TO_KERNEL
 
 kvmppc_call_hv_entry:
-	ld	r4, HSTATE_KVM_VCPU(r13)
+	/* Enter guest. */
 	bl	kvmppc_hv_entry
 
 	/* Back from guest - restore host state and return to caller */

@@ -352,9 +352,7 @@ kvm_secondary_got_guest:
 	mtspr	SPRN_LDBAR, r0
 	isync
 63:
-	/* Order load of vcpu after load of vcore */
-	lwsync

Where did this barrier go?

I don't see that it's covered by any existing barriers in
kvmppc_hv_entry, and you don't add it back anywhere. 

My concept about this is that since now the call to kvmppc_hv_entry
is first taken before the load to r4 shouldn't the pending load in the
pipeline of the HSTATE_KVM_VCORE as per the earlier comment be ordered anyway
before-hand ?

No.
 

quoted

Or do you mean to say that pending loads may not be
cleared/flushed across the "bl <funcname>" boundary ?

Right.

The "bl" imposes no ordering on loads before or after it.

In general nothing orders two independant loads, other than a barrier.

There's some docs on barriers here:

  https://www.kernel.org/doc/Documentation/memory-barriers.txt

Though admittedly it is pretty dense.

cheers