Claudio Carvalho <cclaudio@xxxxxxxxxxxxx> writes:
> On 7/11/19 9:57 AM, Michael Ellerman wrote:
>>> static pmd_t *get_pmd_from_cache(struct mm_struct *mm)
>>> diff --git a/arch/powerpc/mm/book3s64/radix_pgtable.c b/arch/powerpc/mm/book3s64/radix_pgtable.c
>>> index 8904aa1243d8..da6a6b76a040 100644
>>> --- a/arch/powerpc/mm/book3s64/radix_pgtable.c
>>> +++ b/arch/powerpc/mm/book3s64/radix_pgtable.c
>>> @@ -656,8 +656,10 @@ void radix__early_init_mmu_secondary(void)
>>> lpcr = mfspr(SPRN_LPCR);
>>> mtspr(SPRN_LPCR, lpcr | LPCR_UPRT | LPCR_HR);
>>>
>>> - mtspr(SPRN_PTCR,
>>> - __pa(partition_tb) | (PATB_SIZE_SHIFT - 12));
>>> + if (!firmware_has_feature(FW_FEATURE_ULTRAVISOR))
>>> + mtspr(SPRN_PTCR, __pa(partition_tb) |
>>> + (PATB_SIZE_SHIFT - 12));
>>> +
>>> radix_init_amor();
>>> }
>>>
>>> @@ -673,7 +675,8 @@ void radix__mmu_cleanup_all(void)
>>> if (!firmware_has_feature(FW_FEATURE_LPAR)) {
>>> lpcr = mfspr(SPRN_LPCR);
>>> mtspr(SPRN_LPCR, lpcr & ~LPCR_UPRT);
>>> - mtspr(SPRN_PTCR, 0);
>>> + if (!firmware_has_feature(FW_FEATURE_ULTRAVISOR))
>>> + mtspr(SPRN_PTCR, 0);
>>> powernv_set_nmmu_ptcr(0);
>>> radix__flush_tlb_all();
>>> }
>> There's four of these case where we skip touching the PTCR, which is
>> right on the borderline of warranting an accessor. I guess we can do it
>> as a cleanup later.
>
> I agree.
>
> Since the kernel doesn't need to access a big number of ultravisor
> privileged registers, maybe we can define mtspr_<reg> and mfspr_<reg>
> inline functions that in ultravisor.h that skip touching the register if an
> ultravisor is present and and the register is ultravisor privileged. Thus,
> we don't need to replicate comments and that also would make it easier for
> developers to know what are the ultravisor privileged registers.
>
> Something like this:
>
> --- a/arch/powerpc/include/asm/ultravisor.h
> +++ b/arch/powerpc/include/asm/ultravisor.h
> @@ -10,10 +10,21 @@
>
> #include <asm/ultravisor-api.h>
> #include <asm/asm-prototypes.h>
> +#include <asm/reg.h>
>
> int early_init_dt_scan_ultravisor(unsigned long node, const char *uname,
> int depth, void *data);
>
> +static inline void mtspr_ptcr(unsigned long val)
> +{
> + /*
> + * If the ultravisor firmware is present, it maintains the partition
> + * table. PTCR becomes ultravisor privileged only for writing.
> + */
> + if (!firmware_has_feature(FW_FEATURE_ULTRAVISOR))
> + mtspr(SPRN_PTCR, val);
> +}
+
> static inline int uv_register_pate(u64 lpid, u64 dw0, u64 dw1)
> {
> return ucall_norets(UV_WRITE_PATE, lpid, dw0, dw1);
> diff --git a/arch/powerpc/mm/book3s64/pgtable.c
> b/arch/powerpc/mm/book3s64/pgtable.c
> index e1bbc48e730f..25156f9dfde8 100644
> --- a/arch/powerpc/mm/book3s64/pgtable.c
> +++ b/arch/powerpc/mm/book3s64/pgtable.c
> @@ -220,7 +220,7 @@ void __init mmu_partition_table_init(void)
> * 64 K size.
> */
> ptcr = __pa(partition_tb) | (PATB_SIZE_SHIFT - 12);
> - mtspr(SPRN_PTCR, ptcr);
> + mtspr_ptcr(ptcr);
> powernv_set_nmmu_ptcr(ptcr);
> }
>
> What do you think?
I don't think that's actually clearer.
If the logic was always:
if (ultravisor)
do_ucall()
else
mtspr()
Then a wrapper called eg. set_ptcr() would make sense.
But because in some cases you do a ucall and some you don't, I don't
think it helps to hide that in an accessor like above.
That is confusing to a reader who sees all this code to setup a value
and then the write to PTCR does nothing.
And in fact you didn't explain why it's OK for those cases to not do the
write at all.
> An alternative could be to change the mtspr() and mfspr() macros as we
> proposed in the v1, but access to non-ultravisor privileged registers would
> be performance impacted because we always would need to check if the
> register is one of the few ultravisor registers that the kernel needs to
> access.
Yeah that and it would be very confusing to a reader who sees:
ptcr = ...;
mtspr(SPRN_PTCR, ptcr);
...
And then they discover the mtspr does *nothing* when the Ultravisor is
enabled.
cheers
