KVM: PPC: Book3S HV: Tolerate treclaim. in fake-suspend mode changing registers

POWER9 DD2.2 and 2.3 hardware implements a "fake-suspend" mode where
certain TM instructions executed in HV=0 mode cause softpatch interrupts
so the hypervisor can emulate them and prevent problematic processor
conditions. In this fake-suspend mode, the treclaim. instruction does
not modify registers.

Unfortunately the rfscv instruction executed by the guest do not
generate softpatch interrupts, which can cause the hypervisor to lose
track of the fake-suspend mode, and it can execute this treclaim. while
not in fake-suspend mode. This modifies GPRs and crashes the hypervisor.

It's not trivial to disable scv in the guest with HFSCR now, because
they assume a POWER9 has scv available. So this fix saves and restores
checkpointed registers across the treclaim.

Fixes: 7854f75 ("KVM: PPC: Book3S: Rework TM save/restore code and make it C-callable")
Signed-off-by: Nicholas Piggin <[email protected]>
Signed-off-by: Michael Ellerman <[email protected]>
Link: https://lore.kernel.org/r/[email protected]

Author: npiggin

arch/powerpc/kvm/book3s_hv_rmhandlers.S

@@ -2536,7 +2536,7 @@ END_FTR_SECTION_IFCLR(CPU_FTR_P9_TM_HV_ASSIST)
 	/* The following code handles the fake_suspend = 1 case */
 	mflr	r0
 	std	r0, PPC_LR_STKOFF(r1)
-	stdu	r1, -PPC_MIN_STKFRM(r1)
+	stdu	r1, -TM_FRAME_SIZE(r1)
 
 	/* Turn on TM. */
 	mfmsr	r8
@@ -2551,10 +2551,42 @@ BEGIN_FTR_SECTION
 END_FTR_SECTION_IFSET(CPU_FTR_P9_TM_XER_SO_BUG)
 	nop
 
+	/*
+	 * It's possible that treclaim. may modify registers, if we have lost
+	 * track of fake-suspend state in the guest due to it using rfscv.
+	 * Save and restore registers in case this occurs.
+	 */
+	mfspr	r3, SPRN_DSCR
+	mfspr	r4, SPRN_XER
+	mfspr	r5, SPRN_AMR
+	/* SPRN_TAR would need to be saved here if the kernel ever used it */
+	mfcr	r12
+	SAVE_NVGPRS(r1)
+	SAVE_GPR(2, r1)
+	SAVE_GPR(3, r1)
+	SAVE_GPR(4, r1)
+	SAVE_GPR(5, r1)
+	stw	r12, 8(r1)
+	std	r1, HSTATE_HOST_R1(r13)
+
 	/* We have to treclaim here because that's the only way to do S->N */
 	li	r3, TM_CAUSE_KVM_RESCHED
 	TRECLAIM(R3)
 
+	GET_PACA(r13)
+	ld	r1, HSTATE_HOST_R1(r13)
+	REST_GPR(2, r1)
+	REST_GPR(3, r1)
+	REST_GPR(4, r1)
+	REST_GPR(5, r1)
+	lwz	r12, 8(r1)
+	REST_NVGPRS(r1)
+	mtspr	SPRN_DSCR, r3
+	mtspr	SPRN_XER, r4
+	mtspr	SPRN_AMR, r5
+	mtcr	r12
+	HMT_MEDIUM
+
 	/*
 	 * We were in fake suspend, so we are not going to save the
 	 * register state as the guest checkpointed state (since
@@ -2582,7 +2614,7 @@ END_FTR_SECTION_IFSET(CPU_FTR_P9_TM_XER_SO_BUG)
 	std	r5, VCPU_TFHAR(r9)
 	std	r6, VCPU_TFIAR(r9)
 
-	addi	r1, r1, PPC_MIN_STKFRM
+	addi	r1, r1, TM_FRAME_SIZE
 	ld	r0, PPC_LR_STKOFF(r1)
 	mtlr	r0
 	blr