Re: [kvm-unit-tests PATCH 0/4] RFC: Minor arm/arm64 MMU fixes and checks

[Andrew Jones <drjones@redhat.com>]

On Fri, Mar 19, 2021 at 12:24:10PM +0000, Nikos Nikoleris wrote:
> Prior to this set of fixes, the code in setup() which we call to
> initialize the system has a circular dependency. cpu_init()
> (eventually) calls spin_lock() and __mmu_enabled(). However, at this
> point, __mmu_enabled() may have undefined behavior as we haven't
> initialized the current thread_info (cpu field). Moving
> thread_info_init() above cpu_init() is not possible as it relies on
> mpidr_to_cpu() which won't return the right results before cpu_init().
> In addition, mem_init() also calls __mmu_enabled() and therefore
> suffers from the same problem. Right now, everything works as
> thread_info maps to memory which is implicitly initialized to 0 (cpu =
> 0) and makes the assumption that the cpu that runs setup() will be the
> first cpu in the DT.
> 
> This set of patches changes the code slightly and avoids this
> assumptions. In addition, it adds assertions to catch problems like
> the above. The current solution relies on the thread_info() of the cpu
> that setup() run to be initialized to zero (should we make it
> explicit?).
> 
> There are a couple of options I considered in addressing this issue
> which didn't seem satisfactory:
> 
> - Change the mmu_disabled_count global variable to mmu_enabled_count
>   to count the number of active mmu's and bypass __mmu_enabled() when
>   it's 0. This is a global variable and at the momement all write to
>   it are protected by a lock but it's rather fragile and could easily
>   cause issues in the future.
> - Explicitly initialize current_thread_info()->cpu = 0 in setup()
>   before anything else or make the first call of thread_info_init() a
>   special case and avoid looking up mpidr_to_cpu(). This way we can
>   move thread_info_init() to the top of setup(). If the CPU setup is
>   running on is not the first that appears in the DT then this
>   solution won't work.
> 
> Thanks,
> 
> Nikos
> 
> Nikos Nikoleris (4):
>   arm/arm64: Avoid calling cpumask_test_cpu for CPUs above nr_cpu
>   arm/arm64: Read system registers to get the state of the MMU
>   arm/arm64: Track whether thread_info has been initialized
>   arm/arm64: Add sanity checks to the cpumask API
> 
>  lib/arm/asm/cpumask.h     |  7 ++++++-
>  lib/arm/asm/mmu-api.h     |  7 +------
>  lib/arm/asm/processor.h   |  7 +++++++
>  lib/arm/asm/thread_info.h |  1 +
>  lib/arm64/asm/processor.h |  1 +
>  lib/arm/mmu.c             | 16 ++++++++--------
>  lib/arm/processor.c       | 10 ++++++++--
>  lib/arm64/processor.c     | 10 ++++++++--
>  8 files changed, 40 insertions(+), 19 deletions(-)
> 
> -- 
> 2.25.1
>

Hi Nikos,

So, I like patches 1, 2, and 4. I think 3 can be dropped with the
addition of "arm/arm64: Zero BSS and stack at startup". Would you
agree? I've applied all the updated commit messages and all of
Alexandru's suggestions to 2. Patch 2 now looks like the diff below.

Alexandru, are you satisfied with 1, 2, and 4? If so, please let me
know and I'll add r-b's for you before queuing.

Thanks,
drew


diff --git a/lib/arm/asm/mmu-api.h b/lib/arm/asm/mmu-api.h
index 3d04d0312622..05fc12b5afb8 100644
--- a/lib/arm/asm/mmu-api.h
+++ b/lib/arm/asm/mmu-api.h
@@ -5,12 +5,7 @@
 #include <stdbool.h>
 
 extern pgd_t *mmu_idmap;
-extern unsigned int mmu_disabled_cpu_count;
-extern bool __mmu_enabled(void);
-static inline bool mmu_enabled(void)
-{
-	return mmu_disabled_cpu_count == 0 || __mmu_enabled();
-}
+extern bool mmu_enabled(void);
 extern void mmu_mark_enabled(int cpu);
 extern void mmu_mark_disabled(int cpu);
 extern void mmu_enable(pgd_t *pgtable);
diff --git a/lib/arm/asm/processor.h b/lib/arm/asm/processor.h
index 273366d1fe1c..6b762ad060eb 100644
--- a/lib/arm/asm/processor.h
+++ b/lib/arm/asm/processor.h
@@ -67,11 +67,13 @@ extern int mpidr_to_cpu(uint64_t mpidr);
 	((mpidr >> MPIDR_LEVEL_SHIFT(level)) & 0xff)
 
 extern void start_usr(void (*func)(void *arg), void *arg, unsigned long sp_usr);
+extern bool __mmu_enabled(void);
 extern bool is_user(void);
 
 #define CNTVCT		__ACCESS_CP15_64(1, c14)
 #define CNTFRQ		__ACCESS_CP15(c14, 0, c0, 0)
 #define CTR		__ACCESS_CP15(c0, 0, c0, 1)
+#define SCTRL		__ACCESS_CP15(c1, 0, c0, 0)
 
 static inline u64 get_cntvct(void)
 {
diff --git a/lib/arm/mmu.c b/lib/arm/mmu.c
index a1862a55db8b..15eef007f256 100644
--- a/lib/arm/mmu.c
+++ b/lib/arm/mmu.c
@@ -24,13 +24,10 @@ extern unsigned long etext;
 pgd_t *mmu_idmap;
 
 /* CPU 0 starts with disabled MMU */
-static cpumask_t mmu_disabled_cpumask = { {1} };
-unsigned int mmu_disabled_cpu_count = 1;
+static cpumask_t mmu_enabled_cpumask;
 
-bool __mmu_enabled(void)
+bool mmu_enabled(void)
 {
-	int cpu = current_thread_info()->cpu;
-
 	/*
 	 * mmu_enabled is called from places that are guarding the
 	 * use of exclusive ops (which require the mmu to be enabled).
@@ -38,19 +35,22 @@ bool __mmu_enabled(void)
 	 * spinlock, atomic bitop, etc., otherwise we'll recurse.
 	 * [cpumask_]test_bit is safe though.
 	 */
-	return !cpumask_test_cpu(cpu, &mmu_disabled_cpumask);
+	if (is_user()) {
+		int cpu = current_thread_info()->cpu;
+		return cpumask_test_cpu(cpu, &mmu_enabled_cpumask);
+	}
+
+	return __mmu_enabled();
 }
 
 void mmu_mark_enabled(int cpu)
 {
-	if (cpumask_test_and_clear_cpu(cpu, &mmu_disabled_cpumask))
-		--mmu_disabled_cpu_count;
+	cpumask_set_cpu(cpu, &mmu_enabled_cpumask);
 }
 
 void mmu_mark_disabled(int cpu)
 {
-	if (!cpumask_test_and_set_cpu(cpu, &mmu_disabled_cpumask))
-		++mmu_disabled_cpu_count;
+	cpumask_clear_cpu(cpu, &mmu_enabled_cpumask);
 }
 
 extern void asm_mmu_enable(phys_addr_t pgtable);
diff --git a/lib/arm/processor.c b/lib/arm/processor.c
index 773337e6d3b7..9d5759686b73 100644
--- a/lib/arm/processor.c
+++ b/lib/arm/processor.c
@@ -145,3 +145,8 @@ bool is_user(void)
 {
 	return current_thread_info()->flags & TIF_USER_MODE;
 }
+
+bool __mmu_enabled(void)
+{
+	return read_sysreg(SCTRL) & CR_M;
+}
diff --git a/lib/arm64/asm/processor.h b/lib/arm64/asm/processor.h
index 771b2d1e0c94..8e2037e1a43e 100644
--- a/lib/arm64/asm/processor.h
+++ b/lib/arm64/asm/processor.h
@@ -98,6 +98,7 @@ extern int mpidr_to_cpu(uint64_t mpidr);
 
 extern void start_usr(void (*func)(void *arg), void *arg, unsigned long sp_usr);
 extern bool is_user(void);
+extern bool __mmu_enabled(void);
 
 static inline u64 get_cntvct(void)
 {
diff --git a/lib/arm64/processor.c b/lib/arm64/processor.c
index 2a024e3f4e9d..ef558625e284 100644
--- a/lib/arm64/processor.c
+++ b/lib/arm64/processor.c
@@ -257,3 +257,8 @@ bool is_user(void)
 {
 	return current_thread_info()->flags & TIF_USER_MODE;
 }
+
+bool __mmu_enabled(void)
+{
+	return read_sysreg(sctlr_el1) & SCTLR_EL1_M;
+}