linux-kernel - Re: [PATCH v9 5/8] KVM: arm64: Support PUD hugepage in stage2_is

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Date:   Wed, 5 Dec 2018 17:57:51 +0000
From:   Suzuki K Poulose <suzuki.poulose@....com>
To:     Christoffer Dall <christoffer.dall@....com>,
        Punit Agrawal <punit.agrawal@....com>
Cc:     kvmarm@...ts.cs.columbia.edu, marc.zyngier@....com,
        will.deacon@....com, linux-kernel@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, punitagrawal@...il.com,
        Russell King <linux@...linux.org.uk>,
        Catalin Marinas <catalin.marinas@....com>
Subject: Re: [PATCH v9 5/8] KVM: arm64: Support PUD hugepage in
 stage2_is_exec()



On 01/11/2018 13:38, Christoffer Dall wrote:
> On Wed, Oct 31, 2018 at 05:57:42PM +0000, Punit Agrawal wrote:
>> In preparation for creating PUD hugepages at stage 2, add support for
>> detecting execute permissions on PUD page table entries. Faults due to
>> lack of execute permissions on page table entries is used to perform
>> i-cache invalidation on first execute.
>>
>> Provide trivial implementations of arm32 helpers to allow sharing of
>> code.
>>
>> Signed-off-by: Punit Agrawal <punit.agrawal@....com>
>> Reviewed-by: Suzuki K Poulose <suzuki.poulose@....com>
>> Cc: Christoffer Dall <christoffer.dall@....com>
>> Cc: Marc Zyngier <marc.zyngier@....com>
>> Cc: Russell King <linux@...linux.org.uk>
>> Cc: Catalin Marinas <catalin.marinas@....com>
>> Cc: Will Deacon <will.deacon@....com>
>> ---
>>   arch/arm/include/asm/kvm_mmu.h         |  6 +++
>>   arch/arm64/include/asm/kvm_mmu.h       |  5 +++
>>   arch/arm64/include/asm/pgtable-hwdef.h |  2 +
>>   virt/kvm/arm/mmu.c                     | 53 +++++++++++++++++++++++---
>>   4 files changed, 61 insertions(+), 5 deletions(-)
>>
>> diff --git a/arch/arm/include/asm/kvm_mmu.h b/arch/arm/include/asm/kvm_mmu.h
>> index 37bf85d39607..839a619873d3 100644
>> --- a/arch/arm/include/asm/kvm_mmu.h
>> +++ b/arch/arm/include/asm/kvm_mmu.h
>> @@ -102,6 +102,12 @@ static inline bool kvm_s2pud_readonly(pud_t *pud)
>>   	return false;
>>   }
>>   
>> +static inline bool kvm_s2pud_exec(pud_t *pud)
>> +{
>> +	BUG();
> 
> nit: I think this should be WARN() now :)
> 
>> +	return false;
>> +}
>> +
>>   static inline pte_t kvm_s2pte_mkwrite(pte_t pte)
>>   {
>>   	pte_val(pte) |= L_PTE_S2_RDWR;
>> diff --git a/arch/arm64/include/asm/kvm_mmu.h b/arch/arm64/include/asm/kvm_mmu.h
>> index 8da6d1b2a196..c755b37b3f92 100644
>> --- a/arch/arm64/include/asm/kvm_mmu.h
>> +++ b/arch/arm64/include/asm/kvm_mmu.h
>> @@ -261,6 +261,11 @@ static inline bool kvm_s2pud_readonly(pud_t *pudp)
>>   	return kvm_s2pte_readonly((pte_t *)pudp);
>>   }
>>   
>> +static inline bool kvm_s2pud_exec(pud_t *pudp)
>> +{
>> +	return !(READ_ONCE(pud_val(*pudp)) & PUD_S2_XN);
>> +}
>> +
>>   #define hyp_pte_table_empty(ptep) kvm_page_empty(ptep)
>>   
>>   #ifdef __PAGETABLE_PMD_FOLDED
>> diff --git a/arch/arm64/include/asm/pgtable-hwdef.h b/arch/arm64/include/asm/pgtable-hwdef.h
>> index 1d7d8da2ef9b..336e24cddc87 100644
>> --- a/arch/arm64/include/asm/pgtable-hwdef.h
>> +++ b/arch/arm64/include/asm/pgtable-hwdef.h
>> @@ -193,6 +193,8 @@
>>   #define PMD_S2_RDWR		(_AT(pmdval_t, 3) << 6)   /* HAP[2:1] */
>>   #define PMD_S2_XN		(_AT(pmdval_t, 2) << 53)  /* XN[1:0] */
>>   
>> +#define PUD_S2_XN		(_AT(pudval_t, 2) << 53)  /* XN[1:0] */
>> +
>>   /*
>>    * Memory Attribute override for Stage-2 (MemAttr[3:0])
>>    */
>> diff --git a/virt/kvm/arm/mmu.c b/virt/kvm/arm/mmu.c
>> index 1c669c3c1208..8e44dccd1b47 100644
>> --- a/virt/kvm/arm/mmu.c
>> +++ b/virt/kvm/arm/mmu.c
>> @@ -1083,23 +1083,66 @@ static int stage2_set_pmd_huge(struct kvm *kvm, struct kvm_mmu_memory_cache
>>   	return 0;
>>   }
>>   
>> -static bool stage2_is_exec(struct kvm *kvm, phys_addr_t addr)
>> +/*
>> + * stage2_get_leaf_entry - walk the stage2 VM page tables and return
>> + * true if a valid and present leaf-entry is found. A pointer to the
>> + * leaf-entry is returned in the appropriate level variable - pudpp,
>> + * pmdpp, ptepp.
>> + */
>> +static bool stage2_get_leaf_entry(struct kvm *kvm, phys_addr_t addr,
>> +				  pud_t **pudpp, pmd_t **pmdpp, pte_t **ptepp)
> 
> Do we need this type madness or could this just return a u64 pointer
> (NULL if nothing is found) and pass that to kvm_s2pte_exec (because we
> know it's the same bit we need to check regardless of the pgtable level
> on both arm and arm64)?
> 
> Or do we consider that bad for some reason?

Practically, yes the bit positions are same and thus we should be able
to do this assuming that it is just a pte. When we get to independent stage2
pgtable implementation which treats all page table entries as a single type
with a level information, we should be able to get rid of these.
But since we have followed the Linux way of page-table manipulation where we
have "level" specific accessors. The other option is open code the walking
sequence from the pgd to the leaf entry everywhere.

I am fine with changing this code, if you like.

Cheers
Suzuki