Re: [RFC PATCH v4 1/5] RISC-V: Detect and Enable Svadu Extension Support

[Alexandre Ghiti <alex@ghiti.fr>]

Hi Andrew,

On 30/05/2024 10:47, Andrew Jones wrote:
> On Thu, May 30, 2024 at 10:19:12AM GMT, Alexandre Ghiti wrote:
>> Hi Yong-Xuan,
>>
>> On 27/05/2024 18:25, Andrew Jones wrote:
>>> On Fri, May 24, 2024 at 06:33:01PM GMT, Yong-Xuan Wang wrote:
>>>> Svadu is a RISC-V extension for hardware updating of PTE A/D bits.
>>>>
>>>> In this patch we detect Svadu extension support from DTB and enable it
>>>> with SBI FWFT extension. Also we add arch_has_hw_pte_young() to enable
>>>> optimization in MGLRU and __wp_page_copy_user() if Svadu extension is
>>>> available.
>>
>> So we talked about this yesterday during the linux-riscv patchwork meeting.
>> We came to the conclusion that we should not wait for the SBI FWFT extension
>> to enable Svadu but instead, it should be enabled by default by openSBI if
>> the extension is present in the device tree. This is because we did not find
>> any backward compatibility issues, meaning that enabling Svadu should not
>> break any S-mode software.
> Unfortunately I joined yesterday's patchwork call late and missed this
> discussion. I'm still not sure how we avoid concerns with S-mode software
> expecting exceptions by purposely not setting A/D bits, but then not
> getting those exceptions.


Most other architectures implement hardware A/D updates, so I don't see 
what's specific in riscv. In addition, if an OS really needs the 
exceptions, it can always play with the page table permissions to 
achieve such behaviour.


>
>> This is what you did in your previous versions of
>> this patchset so the changes should be easy. This behaviour must be added to
>> the dtbinding description of the Svadu extension.
>>
>> Another thing that we discussed yesterday. There exist 2 schemes to manage
>> the A/D bits updates, Svade and Svadu. If a platform supports both
>> extensions and both are present in the device tree, it is M-mode firmware's
>> responsibility to provide a "sane" device tree to the S-mode software,
>> meaning the device tree can not contain both extensions. And because on such
>> platforms, Svadu is more performant than Svade, Svadu should be enabled by
>> the M-mode firmware and only Svadu should be present in the device tree.
> I'm not sure firmware will be able to choose svadu when it's available.
> For example, platforms which want to conform to the upcoming "Server
> Platform" specification must also conform to the RVA23 profile, which
> mandates Svade and lists Svadu as an optional extension. This implies to
> me that S-mode should be boot with both svade and svadu in the DT and with
> svade being the active one. Then, S-mode can choose to request switching
> to svadu with FWFT.


The problem is that FWFT is not there and won't be there for ~1y 
(according to Anup). So in the meantime, we prevent all uarchs that 
support Svadu to take advantage of this.


>
> Thanks,
> drew
>
>> I hope that clearly explains what we discussed yesterday, let me know if you
>> (or anyone else) need more explanations. If no one is opposed to this
>> solution, do you think you can implement this behaviour? If not, I can deal
>> with it, just let me know.
>>
>> Thanks
>>
>>
>>>> Co-developed-by: Jinyu Tang <tjytimi@163.com>
>>>> Signed-off-by: Jinyu Tang <tjytimi@163.com>
>>>> Signed-off-by: Yong-Xuan Wang <yongxuan.wang@sifive.com>
>>>> Reviewed-by: Conor Dooley <conor.dooley@microchip.com>
>>>> Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
>>> I think this patch changed too much to keep r-b's. We didn't have the
>>> FWFT part before.
>>>
>>>> ---
>>>>    arch/riscv/Kconfig               |  1 +
>>>>    arch/riscv/include/asm/csr.h     |  1 +
>>>>    arch/riscv/include/asm/hwcap.h   |  1 +
>>>>    arch/riscv/include/asm/pgtable.h |  8 +++++++-
>>>>    arch/riscv/kernel/cpufeature.c   | 11 +++++++++++
>>>>    5 files changed, 21 insertions(+), 1 deletion(-)
>>>>
>>>> diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
>>>> index be09c8836d56..30fa558ee284 100644
>>>> --- a/arch/riscv/Kconfig
>>>> +++ b/arch/riscv/Kconfig
>>>> @@ -34,6 +34,7 @@ config RISCV
>>>>    	select ARCH_HAS_PMEM_API
>>>>    	select ARCH_HAS_PREPARE_SYNC_CORE_CMD
>>>>    	select ARCH_HAS_PTE_SPECIAL
>>>> +	select ARCH_HAS_HW_PTE_YOUNG
>>>>    	select ARCH_HAS_SET_DIRECT_MAP if MMU
>>>>    	select ARCH_HAS_SET_MEMORY if MMU
>>>>    	select ARCH_HAS_STRICT_KERNEL_RWX if MMU && !XIP_KERNEL
>>>> diff --git a/arch/riscv/include/asm/csr.h b/arch/riscv/include/asm/csr.h
>>>> index 2468c55933cd..2ac270ad4acd 100644
>>>> --- a/arch/riscv/include/asm/csr.h
>>>> +++ b/arch/riscv/include/asm/csr.h
>>>> @@ -194,6 +194,7 @@
>>>>    /* xENVCFG flags */
>>>>    #define ENVCFG_STCE			(_AC(1, ULL) << 63)
>>>>    #define ENVCFG_PBMTE			(_AC(1, ULL) << 62)
>>>> +#define ENVCFG_ADUE			(_AC(1, ULL) << 61)
>>>>    #define ENVCFG_CBZE			(_AC(1, UL) << 7)
>>>>    #define ENVCFG_CBCFE			(_AC(1, UL) << 6)
>>>>    #define ENVCFG_CBIE_SHIFT		4
>>>> diff --git a/arch/riscv/include/asm/hwcap.h b/arch/riscv/include/asm/hwcap.h
>>>> index e17d0078a651..8d539e3f4e11 100644
>>>> --- a/arch/riscv/include/asm/hwcap.h
>>>> +++ b/arch/riscv/include/asm/hwcap.h
>>>> @@ -81,6 +81,7 @@
>>>>    #define RISCV_ISA_EXT_ZTSO		72
>>>>    #define RISCV_ISA_EXT_ZACAS		73
>>>>    #define RISCV_ISA_EXT_XANDESPMU		74
>>>> +#define RISCV_ISA_EXT_SVADU		75
>>>>    #define RISCV_ISA_EXT_XLINUXENVCFG	127
>>>> diff --git a/arch/riscv/include/asm/pgtable.h b/arch/riscv/include/asm/pgtable.h
>>>> index 9f8ea0e33eb1..1f1b326ccf63 100644
>>>> --- a/arch/riscv/include/asm/pgtable.h
>>>> +++ b/arch/riscv/include/asm/pgtable.h
>>>> @@ -117,6 +117,7 @@
>>>>    #include <asm/tlbflush.h>
>>>>    #include <linux/mm_types.h>
>>>>    #include <asm/compat.h>
>>>> +#include <asm/cpufeature.h>
>>>>    #define __page_val_to_pfn(_val)  (((_val) & _PAGE_PFN_MASK) >> _PAGE_PFN_SHIFT)
>>>> @@ -285,7 +286,6 @@ static inline pte_t pud_pte(pud_t pud)
>>>>    }
>>>>    #ifdef CONFIG_RISCV_ISA_SVNAPOT
>>>> -#include <asm/cpufeature.h>
>>>>    static __always_inline bool has_svnapot(void)
>>>>    {
>>>> @@ -621,6 +621,12 @@ static inline pgprot_t pgprot_writecombine(pgprot_t _prot)
>>>>    	return __pgprot(prot);
>>>>    }
>>>> +#define arch_has_hw_pte_young arch_has_hw_pte_young
>>>> +static inline bool arch_has_hw_pte_young(void)
>>>> +{
>>>> +	return riscv_has_extension_unlikely(RISCV_ISA_EXT_SVADU);
>>>> +}
>>>> +
>>>>    /*
>>>>     * THP functions
>>>>     */
>>>> diff --git a/arch/riscv/kernel/cpufeature.c b/arch/riscv/kernel/cpufeature.c
>>>> index 3ed2359eae35..b023908c5932 100644
>>>> --- a/arch/riscv/kernel/cpufeature.c
>>>> +++ b/arch/riscv/kernel/cpufeature.c
>>>> @@ -93,6 +93,16 @@ static bool riscv_isa_extension_check(int id)
>>>>    			return false;
>>>>    		}
>>>>    		return true;
>>>> +	case RISCV_ISA_EXT_SVADU:
>>>> +		if (sbi_probe_extension(SBI_EXT_FWFT) > 0) {
>>> I think we've decided the appropriate way to prove for SBI extensions is
>>> to first ensure the SBI version and then do the probe, like we do for STA
>>> in has_pv_steal_clock()
>>>
>>>> +			struct sbiret ret;
>>>> +
>>>> +			ret = sbi_ecall(SBI_EXT_FWFT, SBI_EXT_FWFT_SET, SBI_FWFT_PTE_AD_HW_UPDATING,
>>>> +					1, 0, 0, 0, 0);
>>>> +
>>>> +			return ret.error == SBI_SUCCESS;
>>>> +		}
>>>> +		return false;
>>>>    	case RISCV_ISA_EXT_INVALID:
>>>>    		return false;
>>>>    	}
>>>> @@ -301,6 +311,7 @@ const struct riscv_isa_ext_data riscv_isa_ext[] = {
>>>>    	__RISCV_ISA_EXT_DATA(ssaia, RISCV_ISA_EXT_SSAIA),
>>>>    	__RISCV_ISA_EXT_DATA(sscofpmf, RISCV_ISA_EXT_SSCOFPMF),
>>>>    	__RISCV_ISA_EXT_DATA(sstc, RISCV_ISA_EXT_SSTC),
>>>> +	__RISCV_ISA_EXT_SUPERSET(svadu, RISCV_ISA_EXT_SVADU, riscv_xlinuxenvcfg_exts),
>>> We do we need XLINUXENVCFG?
>>>
>>> Thanks,
>>> drew
>>>
>>>>    	__RISCV_ISA_EXT_DATA(svinval, RISCV_ISA_EXT_SVINVAL),
>>>>    	__RISCV_ISA_EXT_DATA(svnapot, RISCV_ISA_EXT_SVNAPOT),
>>>>    	__RISCV_ISA_EXT_DATA(svpbmt, RISCV_ISA_EXT_SVPBMT),
>>>> -- 
>>>> 2.17.1
>>>>
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