linux-kernel - RE: [PATCH v3] arm64: Cleanup __cpu_set_tcr

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Message-ID: <09b201da8c0c$95e8d050$c1ba70f0$@samsung.com>
Date: Thu, 11 Apr 2024 21:34:24 +0900
From: "Seongsu Park" <sgsu.park@...sung.com>
To: "'Will Deacon'" <will@...nel.org>
Cc: <catalin.marinas@....com>, <mark.rutland@....com>,
	<linux-arm-kernel@...ts.infradead.org>, <linux-kernel@...r.kernel.org>,
	"'Leem	ChaeHoon'" <infinite.run@...il.com>, "'Gyeonggeon Choi'"
	<gychoi@...dent.42seoul.kr>, "'Soomin Cho'" <to.soomin@...il.com>, "'DaeRo
 Lee'" <skseofh@...il.com>, "'kmasta'" <kmasta.study@...il.com>
Subject: RE: [PATCH v3] arm64: Cleanup __cpu_set_tcr_t0sz()



> 
> On Mon, Apr 08, 2024 at 11:40:16AM +0900, Seongsu Park wrote:
> > In cpu_set_default_tcr_t0sz(), it is an error to shift TCR_T0SZ_OFFSET
> > twice form TCR_T0SZ() and __cpu_set_tcr_t0sz().
> > Since TCR_T0SZ_OFFSET is 0, no error occurred.
> > We need to clarify whether the parameter of __cpu_set_tcr_t0sz is a
> > shifted value or an unshifted value.
> >
> > We have already shifted the value of t0sz in TCR_T0SZ by
TCR_T0SZ_OFFSET.
> > This is necessary for consistency with TCR_T1SZ.
> > Therefore, the parameter of __cpu_set_tcr_t0sz is clarified as a
> > shifted value.
> 
> This commit message needs reworking. I would suggest something like:
> 
>   The T0SZ field of TCR_EL1 occupies bits 0-5 of the register and
>   encodes the virtual address space translated by TTBR0_EL1. When
>   updating the field (for example, because we are switching to/from
>   the idmap page-table), __cpu_set_tcr_t0sz() erroneously treats its
>   't0sz' argument as unshifted, resulting in harmless but confusing
>   double shifts by 0 in the code.
> 
>   Remove the unnecessary shifts.
> 

Thank you for great feedback.
Please check title and description. 
If these are appropriate, I will write the same in v4.

[Title]
arm64: Cleanup __cpu_set_tcr_t0sz()

[Description]
The T0SZ field of TCR_EL1 occupies bits 0-5 of the register and
encodes the virtual address space translated by TTBR0_EL1. When
updating the field, for example because we are switching to/from
the idmap page-table, __cpu_set_tcr_t0sz() erroneously treats its
't0sz' argument as unshifted, resulting in harmless but confusing
double shifts by 0 in the code. Therefore, Remove the unnecessary
shifts.

> > Co-developed-by: Leem ChaeHoon <infinite.run@...il.com>
> > Signed-off-by: Leem ChaeHoon <infinite.run@...il.com>
> > Co-developed-by: Gyeonggeon Choi <gychoi@...dent.42seoul.kr>
> > Signed-off-by: Gyeonggeon Choi <gychoi@...dent.42seoul.kr>
> > Co-developed-by: Soomin Cho <to.soomin@...il.com>
> > Signed-off-by: Soomin Cho <to.soomin@...il.com>
> > Co-developed-by: DaeRo Lee <skseofh@...il.com>
> > Signed-off-by: DaeRo Lee <skseofh@...il.com>
> > Co-developed-by: kmasta <kmasta.study@...il.com>
> > Signed-off-by: kmasta <kmasta.study@...il.com>
> > Signed-off-by: Seongsu Park <sgsu.park@...sung.com>
> 
> Honestly, although it's great that you all meet up to look at the kernel,
> this long list of credits is a little absurd for a trivial patch like
this.
> Please can you decide who did the most work and give them the credit?
> Hopefully there will be future opportunities for you all to contribute!
> 

Okay. I got it.
In v4, I'll leave only Leem ChaeHoon and me.

> > ---
> >
> > v2:
> >  - Condition is updated
> > v3:
> >  - Commit message is updated
> >  - cpu_set_tcr_t0sz macro is added
> >
> > ---
> >  arch/arm64/include/asm/mmu_context.h | 7 ++++---
> >  1 file changed, 4 insertions(+), 3 deletions(-)
> >
> > diff --git a/arch/arm64/include/asm/mmu_context.h
> > b/arch/arm64/include/asm/mmu_context.h
> > index c768d16b81a4..fb603ec7f61f 100644
> > --- a/arch/arm64/include/asm/mmu_context.h
> > +++ b/arch/arm64/include/asm/mmu_context.h
> > @@ -72,15 +72,16 @@ static inline void __cpu_set_tcr_t0sz(unsigned
> > long t0sz)  {
> >  	unsigned long tcr = read_sysreg(tcr_el1);
> >
> > -	if ((tcr & TCR_T0SZ_MASK) >> TCR_T0SZ_OFFSET == t0sz)
> > +	if ((tcr & TCR_T0SZ_MASK) == t0sz)
> >  		return;
> >
> >  	tcr &= ~TCR_T0SZ_MASK;
> > -	tcr |= t0sz << TCR_T0SZ_OFFSET;
> > +	tcr |= t0sz;
> >  	write_sysreg(tcr, tcr_el1);
> >  	isb();
> >  }
> >
> > +#define cpu_set_tcr_t0sz(t0sz)
> 	__cpu_set_tcr_t0sz(TCR_T0SZ(t0sz))
> >  #define cpu_set_default_tcr_t0sz()
> 	__cpu_set_tcr_t0sz(TCR_T0SZ(vabits_actual))
> >  #define cpu_set_idmap_tcr_t0sz()	__cpu_set_tcr_t0sz(idmap_t0sz)
> >
> > @@ -134,7 +135,7 @@ static inline void cpu_install_ttbr0(phys_addr_t
> > ttbr0, unsigned long t0sz)  {
> >  	cpu_set_reserved_ttbr0();
> >  	local_flush_tlb_all();
> > -	__cpu_set_tcr_t0sz(t0sz);
> > +	cpu_set_tcr_t0sz(t0sz);
> 
> Sorry, but this is wrong. Please have a look at how cpu_install_ttbr0() is
> called; specifically how trans_pgd_idmap_page() sets up 't0sz'.
> 
> So I don't think you should change cpu_install_ttbr0() at all and adding a
> cpu_set_tcr_t0sz() macro which calls TCR_T0SZ on the 't0sz' argument is a
> mistake.
> 
> Will

Oops. You're right. My mistake.
In v4, I'll remove this.