Re: [PATCH v2] xen/arm64: Remove READ/WRITE_SYSREG32 helper macros

[Julien Grall <julien@xxxxxxx>]

On 12/07/2021 07:26, Michal Orzel wrote:
> Hi Julien,
Hi Michal,

> On 09.07.2021 17:21, Julien Grall wrote:
> > Hi Michal,
> >
> > On 09/07/2021 13:40, Michal Orzel wrote:
> > > AArch64 system registers are 64bit whereas AArch32 ones
> > > are 32bit or 64bit. MSR/MRS are expecting 64bit values thus
> > > we should get rid of helpers READ/WRITE_SYSREG32
> > > in favour of using READ/WRITE_SYSREG.
> > >
> > > The last place in code making use of READ/WRITE_SYSREG32
> > > on arm64 is in TVM_REG macro defining functions vreg_emulate_<register>.
> > > Implement a macro WRITE_SYSREG_SZ which expands as follows:
> > > -on arm64: WRITE_SYSREG
> > > -on arm32: WRITE_SYSREG{32/64}
> > >
> > > As there are no other places in the code using these helpers
> > > on arm64 - remove them.
> > >
> > > Signed-off-by: Michal Orzel <michal.orzel@xxxxxxx>
> > > ---
> > > Changes since v1:
> > > -implement WRITE_SYSREG_SZ instead of duplicating the TVM_REG
> > > ---
> > >    xen/arch/arm/vcpreg.c               | 12 +++++++++++-
> > >    xen/include/asm-arm/arm64/sysregs.h |  4 ----
> > >    2 files changed, 11 insertions(+), 5 deletions(-)
> > >
> > > diff --git a/xen/arch/arm/vcpreg.c b/xen/arch/arm/vcpreg.c
> > > index f0cdcc8a54..10c4846954 100644
> > > --- a/xen/arch/arm/vcpreg.c
> > > +++ b/xen/arch/arm/vcpreg.c
> > > @@ -47,6 +47,16 @@
> > >     *
> > >     */
> > >    +#ifdef CONFIG_ARM_64
> > > +#define WRITE_SYSREG_SZ(sz, val, sysreg)     WRITE_SYSREG(val, sysreg)
> > I think you want to cast to (uint##sz##_t) to avoid any surprise. I think...
> But the val will always be of type uint##sz##_t because it is passed from ...
> > > +#else
> > > +/*
> > > + * WRITE_SYSREG{32/64} on arm32 is defined as variadic macro which imposes
> > > + * on the below macro to be defined like that as well.
> > > + */
> > > +#define WRITE_SYSREG_SZ(sz, val, sysreg...)  WRITE_SYSREG##sz(val, sysreg)
> > > +#endif
> > > +
> > >    /* The name is passed from the upper macro to workaround macro expansion. */
> > >    #define TVM_REG(sz, func, reg...)                                           \
> > >    static bool func(struct cpu_user_regs *regs, uint##sz##_t *r, bool read)    \
> > > @@ -55,7 +65,7 @@ static bool func(struct cpu_user_regs *regs, uint##sz##_t *r, bool 
> > > read)    \
> ... here(*r argument).
> So I do not think that such casting makes sense e.g
> uint##sz##_t foo;
> WRITE_SYSREG((uint##sz##_t)foo, bar);
I agree that this doesn't make sense for the *current* use. However, 
when writing code, we need to think how one could use it in the future...
When I read the name, I would expect that if I call it with sz == 32, 
then bottom 32-bit value to be written and the top 32-bit zeroed. But 
this is not the case...
You are relying on the developper and reviewer to notice that only 
32-bit value should be passed when sz == 32.
I am not particularly keen on relying on this when a simple cast could 
prevent any future misuse. Alternatively, I would be happy to consider 
checking the type of the value at build time.
Cheers,

--
Julien Grall