[qemu-xen stable-4.18] target/riscv: fix satp_mode_finalize() when satp_mode.supported = 0

[patchbot@xxxxxxx]

commit 1805e05db3c6d92c750427b1c1371f44fa8e4893
Author:     Daniel Henrique Barboza <dbarboza@xxxxxxxxxxxxxxxx>
AuthorDate: Thu Aug 17 12:29:03 2023 -0300
Commit:     Michael Tokarev <mjt@xxxxxxxxxx>
CommitDate: Wed Sep 20 10:18:14 2023 +0300

    target/riscv: fix satp_mode_finalize() when satp_mode.supported = 0
    
    In the same emulated RISC-V host, the 'host' KVM CPU takes 4 times
    longer to boot than the 'rv64' KVM CPU.
    
    The reason is an unintended behavior of riscv_cpu_satp_mode_finalize()
    when satp_mode.supported = 0, i.e. when cpu_init() does not set
    satp_mode_max_supported(). satp_mode_max_from_map(map) does:
    
    31 - __builtin_clz(map)
    
    This means that, if satp_mode.supported = 0, satp_mode_supported_max
    wil be '31 - 32'. But this is C, so satp_mode_supported_max will gladly
    set it to UINT_MAX (4294967295). After that, if the user didn't set a
    satp_mode, set_satp_mode_default_map(cpu) will make
    
    cfg.satp_mode.map = cfg.satp_mode.supported
    
    So satp_mode.map = 0. And then satp_mode_map_max will be set to
    satp_mode_max_from_map(cpu->cfg.satp_mode.map), i.e. also UINT_MAX. The
    guard "satp_mode_map_max > satp_mode_supported_max" doesn't protect us
    here since both are UINT_MAX.
    
    And finally we have 2 loops:
    
            for (int i = satp_mode_map_max - 1; i >= 0; --i) {
    
    Which are, in fact, 2 loops from UINT_MAX -1 to -1. This is where the
    extra delay when booting the 'host' CPU is coming from.
    
    Commit 43d1de32f8 already set a precedence for satp_mode.supported = 0
    in a different manner. We're doing the same here. If supported == 0,
    interpret as 'the CPU wants the OS to handle satp mode alone' and skip
    satp_mode_finalize().
    
    We'll also put a guard in satp_mode_max_from_map() to assert out if map
    is 0 since the function is not ready to deal with it.
    
    Cc: Alexandre Ghiti <alexghiti@xxxxxxxxxxxx>
    Fixes: 6f23aaeb9b ("riscv: Allow user to set the satp mode")
    Signed-off-by: Daniel Henrique Barboza <dbarboza@xxxxxxxxxxxxxxxx>
    Reviewed-by: Andrew Jones <ajones@xxxxxxxxxxxxxxxx>
    Message-ID: <20230817152903.694926-1-dbarboza@xxxxxxxxxxxxxxxx>
    Signed-off-by: Alistair Francis <alistair.francis@xxxxxxx>
    (cherry picked from commit 3a2fc23563885c219c73c8f24318921daf02f3f2)
    Signed-off-by: Michael Tokarev <mjt@xxxxxxxxxx>
---
 target/riscv/cpu.c | 23 ++++++++++++++++++++---
 1 file changed, 20 insertions(+), 3 deletions(-)

diff --git a/target/riscv/cpu.c b/target/riscv/cpu.c
index 1e97473af2..8c0721c715 100644
--- a/target/riscv/cpu.c
+++ b/target/riscv/cpu.c
@@ -282,6 +282,17 @@ static uint8_t satp_mode_from_str(const char 
*satp_mode_str)
 
 uint8_t satp_mode_max_from_map(uint32_t map)
 {
+    /*
+     * 'map = 0' will make us return (31 - 32), which C will
+     * happily overflow to UINT_MAX. There's no good result to
+     * return if 'map = 0' (e.g. returning 0 will be ambiguous
+     * with the result for 'map = 1').
+     *
+     * Assert out if map = 0. Callers will have to deal with
+     * it outside of this function.
+     */
+    g_assert(map > 0);
+
     /* map here has at least one bit set, so no problem with clz */
     return 31 - __builtin_clz(map);
 }
@@ -1043,9 +1054,15 @@ static void riscv_cpu_validate_set_extensions(RISCVCPU 
*cpu, Error **errp)
 static void riscv_cpu_satp_mode_finalize(RISCVCPU *cpu, Error **errp)
 {
     bool rv32 = riscv_cpu_mxl(&cpu->env) == MXL_RV32;
-    uint8_t satp_mode_map_max;
-    uint8_t satp_mode_supported_max =
-                        satp_mode_max_from_map(cpu->cfg.satp_mode.supported);
+    uint8_t satp_mode_map_max, satp_mode_supported_max;
+
+    /* The CPU wants the OS to decide which satp mode to use */
+    if (cpu->cfg.satp_mode.supported == 0) {
+        return;
+    }
+
+    satp_mode_supported_max =
+                    satp_mode_max_from_map(cpu->cfg.satp_mode.supported);
 
     if (cpu->cfg.satp_mode.map == 0) {
         if (cpu->cfg.satp_mode.init == 0) {
--
generated by git-patchbot for /home/xen/git/qemu-xen.git#stable-4.18