Hi,

On 2023-03-22 23:17:35, Michael Ellerman wrote:

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 | 11 ++++++-----
 1 file changed, 6 insertions(+), 5 deletions(-)

diff --git a/arch/powerpc/kvm/book3s_hv_rmhandlers.S b/arch/powerpc/kvm/book3s_hv_rmhandlers.S
index b81ba4ee0521..b61f0b2c677b 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
-	ld	r4, HSTATE_KVM_VCPU(r13)
+	/* Enter guest. */
 	bl	kvmppc_hv_entry
 
 	/* Back from the guest, go back to nap */

@@ -506,7 +504,6 @@ SYM_INNER_LABEL(kvmppc_hv_entry, SYM_L_LOCAL)
 
 	/* Required state:
 	 *
-	 * R4 = vcpu pointer (or NULL)
 	 * MSR = ~IR|DR
 	 * R13 = PACA
 	 * R1 = host R1

@@ -524,6 +521,10 @@ SYM_INNER_LABEL(kvmppc_hv_entry, SYM_L_LOCAL)
 	li	r6, KVM_GUEST_MODE_HOST_HV
 	stb	r6, HSTATE_IN_GUEST(r13)
 
+	/* Order load of vcpu after load of vcore */

Just copying the comment here doesn't work. It doesn't make sense on its
own here, because the VCORE is loaded (again) a few lines below (536).
So as written this comment seems backward vs the code.

The comment would need to expand to explain that the barrier is for the
case where we came from kvm_secondary_got_guest.

Agreed.

quoted

+	lwsync
+	ld	r4, HSTATE_KVM_VCPU(r13)
+
 #ifdef CONFIG_KVM_BOOK3S_HV_P8_TIMING
 	/* Store initial timestamp */
 	cmpdi	r4, 0

But as Nick says I don't think it's worth investing effort in small
tweaks to this code. The risk of introducing bugs is too high for such a
small improvement to the code.

Thanks for trying, but I think this asm code is best left more or less
alone unless we find actual bugs in it - or unless we can make
substantial improvements to it, which would be rewriting in C, or at
least converting to a fully call/return style rather than the current
forest of labels.

I will take patch 1 though, as that's an obvious cleanup and poses no
risk (famous last words :D).

Thanks for taking patch 1. I completely agree that it would not be wise
to introduce even small alterations to stable legacy code. But sending
patches like this and conversing with this mailing list's reviewers
is giving me a good picture of what you are open to accepting at this
stage.

Thanks again!

cheers