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

[Nikos Nikoleris <nikos.nikoleris@xxxxxxx>]

On 23/03/2021 11:51, Andrew Jones wrote:
> On Tue, Mar 23, 2021 at 11:40:37AM +0000, Alexandru Elisei wrote:
> > Hi Drew,
> >
> > On 3/23/21 11:24 AM, Andrew Jones wrote:
> > > 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.
> >
> > The diff looks good, and I agree that we can come back to #3 once we have a better
> > understanding of what needs to be done. Patch #1 is a nice fix for what I imagine
> > was a hard to find bug. Patch #4 also looks correct to me. Yes, please add my
> > Reviewed-by for the series.
> >
> > Thanks,
> >
> > Alex
>
> Thanks, applied 1, 2, and 4 to arm/queue
>
> https://gitlab.com/rhdrjones/kvm-unit-tests/-/commits/arm/queue
>
> Thanks,
> drew

Looks good to me. Thank you both for the reviews and the suggestions!

Thanks,

Nikos

> > > 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;
> > > +}