lib: sbi_domain_context: preserve firmware PMP entries during domain context switch

When SmePMP is enabled, clearing firmware PMP entries during a domain
context switch can temporarily revoke access to OpenSBI’s own code and
data, leading to faults.

Keep firmware PMP entries enabled across switches so firmware regions
remain accessible and executable.

Signed-off-by: Yu-Chien Peter Lin <peter.lin@sifive.com>
Reviewed-by: Anup Patel <anup@brainfault.org>
Link: https://lore.kernel.org/r/20251008084444.3525615-9-peter.lin@sifive.com
Signed-off-by: Anup Patel <anup@brainfault.org>

lib/sbi/sbi_domain_context.c

@@ -122,6 +122,10 @@ static int switch_to_next_domain_context(struct hart_context *ctx,
 
 	/* Reconfigure PMP settings for the new domain */
 	for (int i = 0; i < pmp_count; i++) {
+		/* Don't revoke firmware access permissions */
+		if (sbi_hart_smepmp_is_fw_region(i))
+			continue;
+
 		sbi_platform_pmp_disable(sbi_platform_thishart_ptr(), i);
 		pmp_disable(i);
 	}