Re: [kvm-unit-tests v2 03/10] powerpc: abstract H_CEDE calls into a sleep functions

["Nicholas Piggin" <npiggin@gmail.com>]

On Thu Mar 23, 2023 at 10:12 PM AEST, Thomas Huth wrote:
> On 20/03/2023 08.03, Nicholas Piggin wrote:
> > This consolidates several implementations, and it no longer leaves
> > MSR[EE] enabled after the decrementer interrupt is handled, but
> > rather disables it on return.
> > 
> > The handler no longer allows a continuous ticking, but rather dec
> > has to be re-armed and EE re-enabled (e.g., via H_CEDE hcall) each
> > time.
> > 
> > Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
> > ---
> >   lib/powerpc/asm/handlers.h  |  2 +-
> >   lib/powerpc/asm/ppc_asm.h   |  1 +
> >   lib/powerpc/asm/processor.h |  7 +++++++
> >   lib/powerpc/handlers.c      | 10 ++++-----
> >   lib/powerpc/processor.c     | 42 +++++++++++++++++++++++++++++++++++++
> >   powerpc/sprs.c              |  6 +-----
> >   powerpc/tm.c                | 20 +-----------------
> >   7 files changed, 57 insertions(+), 31 deletions(-)
> > 
> > diff --git a/lib/powerpc/asm/handlers.h b/lib/powerpc/asm/handlers.h
> > index 64ba727..e4a0cd4 100644
> > --- a/lib/powerpc/asm/handlers.h
> > +++ b/lib/powerpc/asm/handlers.h
> > @@ -3,6 +3,6 @@
> >   
> >   #include <asm/ptrace.h>
> >   
> > -void dec_except_handler(struct pt_regs *regs, void *data);
> > +void dec_handler_oneshot(struct pt_regs *regs, void *data);
> >   
> >   #endif /* _ASMPOWERPC_HANDLERS_H_ */
> > diff --git a/lib/powerpc/asm/ppc_asm.h b/lib/powerpc/asm/ppc_asm.h
> > index 1b85f6b..6299ff5 100644
> > --- a/lib/powerpc/asm/ppc_asm.h
> > +++ b/lib/powerpc/asm/ppc_asm.h
> > @@ -36,6 +36,7 @@
> >   #endif /* __BYTE_ORDER__ */
> >   
> >   /* Machine State Register definitions: */
> > +#define MSR_EE_BIT	15			/* External Interrupts Enable */
> >   #define MSR_SF_BIT	63			/* 64-bit mode */
> >   
> >   #endif /* _ASMPOWERPC_PPC_ASM_H */
> > diff --git a/lib/powerpc/asm/processor.h b/lib/powerpc/asm/processor.h
> > index ac001e1..ebfeff2 100644
> > --- a/lib/powerpc/asm/processor.h
> > +++ b/lib/powerpc/asm/processor.h
> > @@ -20,6 +20,8 @@ static inline uint64_t get_tb(void)
> >   
> >   extern void delay(uint64_t cycles);
> >   extern void udelay(uint64_t us);
> > +extern void sleep_tb(uint64_t cycles);
> > +extern void usleep(uint64_t us);
> >   
> >   static inline void mdelay(uint64_t ms)
> >   {
> > @@ -27,4 +29,9 @@ static inline void mdelay(uint64_t ms)
> >   		udelay(1000);
> >   }
> >   
> > +static inline void msleep(uint64_t ms)
> > +{
> > +	usleep(ms * 1000);
> > +}
> > +
> >   #endif /* _ASMPOWERPC_PROCESSOR_H_ */
> > diff --git a/lib/powerpc/handlers.c b/lib/powerpc/handlers.c
> > index c8721e0..296f14f 100644
> > --- a/lib/powerpc/handlers.c
> > +++ b/lib/powerpc/handlers.c
> > @@ -9,15 +9,13 @@
> >   #include <libcflat.h>
> >   #include <asm/handlers.h>
> >   #include <asm/ptrace.h>
> > +#include <asm/ppc_asm.h>
> >   
> >   /*
> >    * Generic handler for decrementer exceptions (0x900)
> > - * Just reset the decrementer back to the value specified when registering the
> > - * handler
> > + * Return with MSR[EE] disabled.
> >    */
> > -void dec_except_handler(struct pt_regs *regs __unused, void *data)
> > +void dec_handler_oneshot(struct pt_regs *regs, void *data)
> >   {
> > -	uint64_t dec = *((uint64_t *) data);
> > -
> > -	asm volatile ("mtdec %0" : : "r" (dec));
> > +	regs->msr &= ~(1UL << MSR_EE_BIT);
> >   }
> > diff --git a/lib/powerpc/processor.c b/lib/powerpc/processor.c
> > index ec85b9d..e77a240 100644
> > --- a/lib/powerpc/processor.c
> > +++ b/lib/powerpc/processor.c
> > @@ -10,6 +10,8 @@
> >   #include <asm/ptrace.h>
> >   #include <asm/setup.h>
> >   #include <asm/barrier.h>
> > +#include <asm/hcall.h>
> > +#include <asm/handlers.h>
> >   
> >   static struct {
> >   	void (*func)(struct pt_regs *, void *data);
> > @@ -54,3 +56,43 @@ void udelay(uint64_t us)
> >   {
> >   	delay((us * tb_hz) / 1000000);
> >   }
> > +
> > +void sleep_tb(uint64_t cycles)
> > +{
> > +	uint64_t start, end, now;
> > +
> > +	start = now = get_tb();
> > +	end = start + cycles;
> > +
> > +	while (end > now) {
> > +		uint64_t left = end - now;
> > +
> > +		/* Could support large decrementer */
> > +		if (left > 0x7fffffff)
> > +			left = 0x7fffffff;
> > +
> > +		asm volatile ("mtdec %0" : : "r" (left));
> > +		handle_exception(0x900, &dec_handler_oneshot, NULL);
>
> Wouldn't it be better to first call handle_exception() before moving 
> something into the decrementer?

It shouldn't really matter, the drecrementer could be anything here
(probably it's already -ve here if it starts at zero). It only matters
when MSR[EE]=1 or we make the H_CEDE call.

I'll add a comment at least.

Thanks,
Nick