powerpc/pesries: Get STF barrier requirement from H_GET_CPU_CHARACTERISTICS

This allows the hypervisor / firmware to describe this workarounds to
the guest.
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20210503130243.891868-4-npiggin@gmail.com

arch/powerpc/include/asm/hvcall.h

@@ -402,6 +402,7 @@
 #define H_CPU_BEHAV_FLUSH_LINK_STACK	(1ull << 57) // IBM bit 6
 #define H_CPU_BEHAV_NO_L1D_FLUSH_ENTRY	(1ull << 56) // IBM bit 7
 #define H_CPU_BEHAV_NO_L1D_FLUSH_UACCESS (1ull << 55) // IBM bit 8
+#define H_CPU_BEHAV_NO_STF_BARRIER	(1ull << 54) // IBM bit 9
 
 /* Flag values used in H_REGISTER_PROC_TBL hcall */
 #define PROC_TABLE_OP_MASK	0x18

arch/powerpc/platforms/pseries/setup.c

@@ -555,6 +555,9 @@ static void init_cpu_char_feature_flags(struct h_cpu_char_result *result)
 	if (result->behaviour & H_CPU_BEHAV_NO_L1D_FLUSH_UACCESS)
 		security_ftr_clear(SEC_FTR_L1D_FLUSH_UACCESS);
 
+	if (result->behaviour & H_CPU_BEHAV_NO_STF_BARRIER)
+		security_ftr_clear(SEC_FTR_STF_BARRIER);
+
 	if (!(result->behaviour & H_CPU_BEHAV_BNDS_CHK_SPEC_BAR))
 		security_ftr_clear(SEC_FTR_BNDS_CHK_SPEC_BAR);
 }