Marcelo Tosatti wrote:
> Forgot to copy you... Ideally all pte updates should be done via the
> paravirt interface.
>
Hm, are you sure?
> +static inline void pte_clear_bit(unsigned int bit, pte_t *ptep)
> +{
> + pte_t pte = *ptep;
> + clear_bit(bit, (unsigned long *)&pte.pte);
> + set_pte(ptep, pte);
> +}
>
This is not a safe transformation. This will lose hardware A/D bit
changes if the pte is part of an active pagetable. Aside from that,
clear_bit is atomic and so is the wrong thing to use on a local
variable; a plain bitmask would do the job.
clear_bit on a PTE is fine, since everyone has to support some level of
trap'n'emulate on pte-level entries (ptep_get_and_clear is very hard to
implement otherwise). Shadow pagetable implementations will do this
naturally, and Xen must do it to allow atomic updates on ptes (also
because it makes late-pinning/early-unpinning a performance win).
> +
> static inline void set_pte_at(struct mm_struct *mm, unsigned long addr,
> pte_t *ptep, pte_t pte)
> {
> Index: linux-2.6-x86-kvm/include/asm-x86/pgtable.h
> ===================================================================
> --- linux-2.6-x86-kvm.orig/include/asm-x86/pgtable.h
> +++ linux-2.6-x86-kvm/include/asm-x86/pgtable.h
> @@ -227,6 +227,8 @@ void native_pagetable_setup_done(pgd_t *
> #define pte_update(mm, addr, ptep) do { } while (0)
> #define pte_update_defer(mm, addr, ptep) do { } while (0)
>
> +#define pte_clear_bit(bit, ptep) clear_bit(bit, (unsigned long
> *)&ptep->pte)
> +
> static inline void paravirt_pagetable_setup_start(pgd_t *base)
> {
> native_pagetable_setup_start(base);
> @@ -302,7 +304,7 @@ static inline void native_set_pte_at(str
> ({ \
> int __changed = !pte_same(*(ptep), entry); \
> if (__changed && dirty) { \
> - *ptep = entry; \
> + set_pte(ptep, entry); \
>
This change is OK, but doesn't really do anything. All hypervisors
allow direct access to ptes.
> pte_update_defer((vma)->vm_mm, (address), (ptep)); \
> flush_tlb_page(vma, address); \
> } \
> @@ -357,7 +359,7 @@ static inline pte_t ptep_get_and_clear_f
> #define __HAVE_ARCH_PTEP_SET_WRPROTECT
> static inline void ptep_set_wrprotect(struct mm_struct *mm, unsigned long
> addr, pte_t *ptep)
> {
> - clear_bit(_PAGE_BIT_RW, (unsigned long *)&ptep->pte);
> + pte_clear_bit(_PAGE_BIT_RW, ptep);
>
This is only valid if ptep_set_wrprotect is never used on active ptes,
which I don't think is the case.
J
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