plat: Implement a method for allocating the auxiliary stack

Add a method that transparently allocates and returns an auxiliary
stack according to CONFIG_UKPLAT_AUXSP_SIZE.

Note that if `libukvmem` is enabled, this stack shall be premapped
as we do not want accesses to this to generate page faults.

Signed-off-by: Sergiu Moga <sergiu@unikraft.io>

diff --git a/include/uk/plat/memory.h b/include/uk/plat/memory.h
index 1a11a665e1c548207a5e4f19fcef20a692958585..08d13d600aac617d773205478c2707621ebfbb97 100644 (file)
--- a/include/uk/plat/memory.h
+++ b/include/uk/plat/memory.h
@@ -33,10 +33,15 @@
 #ifndef __UKPLAT_MEMORY_H__
 #define __UKPLAT_MEMORY_H__
 
+#include <stddef.h>
 #include <uk/essentials.h>
 #include <uk/arch/types.h>
 #include <uk/alloc.h>
 #include <uk/config.h>
+#include <uk/arch/ctx.h>
+#if CONFIG_LIBUKVMEM
+#include <uk/vmem.h>
+#endif /* CONFIG_LIBUKVMEM */
 
 #ifdef __cplusplus
 extern "C" {
@@ -252,6 +257,61 @@ void *ukplat_memregion_alloc(__sz size, int type, __u16 flags);
  */
 int ukplat_mem_init(void);
 
+/* Allocates and returns an auxiliary stack that can be used in emergency cases
+ * such as when switching system call stacks. The size is that given by
+ * CONFIG_AUXSP_SIZE.
+ *
+ * @param a
+ *   The allocator to use for the auxiliary stack
+ * @param vas
+ *   The virtual address space to use for the mapping of the auxiliary stack.
+ *   This should be used in conjunction with CONFIG_LIBUKVMEM to ensure that
+ *   accesses to the auxiliary stack do not generate page faults in more
+ *   fragile system states.
+ * @param auxsp_len
+ *   The custom length of the auxiliary stack. If 0, then
+ *   CONFIG_UKPLAT_AUXSP_SIZE is used instead as the default length.
+ *
+ * @return
+ *   Pointer to the allocated auxiliary stack
+ */
+static inline __uptr ukplat_auxsp_alloc(struct uk_alloc *a,
+#if CONFIG_LIBUKVMEM
+                                       struct uk_vas __maybe_unused *vas,
+#endif /* CONFIG_LIBUKVMEM */
+                                       __sz auxsp_len)
+{
+       void *auxsp = NULL;
+
+       if (!auxsp_len)
+               auxsp_len = ALIGN_UP(CONFIG_UKPLAT_AUXSP_SIZE, UKARCH_SP_ALIGN);
+
+       auxsp = uk_memalign(a, UKARCH_SP_ALIGN, auxsp_len);
+       if (unlikely(!auxsp)) {
+               uk_pr_err("Failed to allocate auxiliary stack\n");
+               return 0;
+       }
+
+#if CONFIG_LIBUKVMEM
+       __uptr auxsp_base = PAGE_ALIGN_DOWN((__uptr)auxsp);
+       int rc;
+
+       /* Ensure the buffer is backed by physical memory */
+       rc = uk_vma_advise(vas,
+                          auxsp_base,
+                          PAGE_ALIGN_UP((__uptr)auxsp + auxsp_len -
+                                        auxsp_base),
+                          UK_VMA_ADV_WILLNEED,
+                          UK_VMA_FLAG_UNINITIALIZED);
+       if (unlikely(rc)) {
+               uk_pr_err("Failed to prematurely map auxiliary stack\n");
+               return 0;
+       }
+#endif /* CONFIG_LIBUKVMEM */
+
+       return (__uptr)ukarch_gen_sp(auxsp, auxsp_len);
+}
+
 #ifdef __cplusplus
 }
 #endif

diff --git a/plat/Config.uk b/plat/Config.uk
index 5cb7c89888225181aa64fabc810c40657b96c17e..5f66da98c9fcf005e6f7ae6027928c07b3197dbd 100644 (file)
--- a/plat/Config.uk
+++ b/plat/Config.uk
@@ -115,3 +115,7 @@ config FPSIMD
        depends on ARCH_ARM_64
        help
                Enable support FPU usage in application
+
+config UKPLAT_AUXSP_SIZE
+       int "Maximum size allocated for the auxiliary stack"
+       default 32768