Re: [Xen-devel] [PATCH 15/18] arm/altp2m: Add altp2m paging mechanism.

[Julien Grall <julien.grall@xxxxxxx>]

(CC Tamas)

Hello Sergej,

On 04/07/2016 12:45, Sergej Proskurin wrote:
> This commit adds the function p2m_altp2m_lazy_copy implementing the
> altp2m paging mechanism. The function p2m_altp2m_lazy_copy lazily copies
> the hostp2m's mapping into the currently active altp2m view on 2nd stage
> instruction or data access violations. Every altp2m violation generates
> a vm_event.
I have been working on clean up the abort path (see [1]). Please rebase 
your code on top of it.
> Signed-off-by: Sergej Proskurin <proskurin@xxxxxxxxxxxxx>
> ---
> Cc: Stefano Stabellini <sstabellini@xxxxxxxxxx>
> Cc: Julien Grall <julien.grall@xxxxxxx>
> ---
[...]

> +/*
> + * If the fault is for a not present entry:
> + *     if the entry in the host p2m has a valid mfn, copy it and retry
> + *     else indicate that outer handler should handle fault
> + *
> + * If the fault is for a present entry:
> + *     indicate that outer handler should handle fault
> + */
> +bool_t p2m_altp2m_lazy_copy(struct vcpu *v, paddr_t gpa,
> +                            unsigned long gva, struct npfec npfec,
> +                            struct p2m_domain **ap2m)
> +{
> +    struct domain *d = v->domain;
> +    struct p2m_domain *hp2m = p2m_get_hostp2m(v->domain);
> +    p2m_type_t p2mt;
> +    xenmem_access_t xma;
> +    paddr_t maddr, mask = 0;
> +    gfn_t gfn = _gfn(paddr_to_pfn(gpa));
> +    unsigned int level;
> +    unsigned long mattr;
> +    int rc = 0;
> +
> +    static const p2m_access_t memaccess[] = {
> +#define ACCESS(ac) [XENMEM_access_##ac] = p2m_access_##ac
> +        ACCESS(n),
> +        ACCESS(r),
> +        ACCESS(w),
> +        ACCESS(rw),
> +        ACCESS(x),
> +        ACCESS(rx),
> +        ACCESS(wx),
> +        ACCESS(rwx),
> +        ACCESS(rx2rw),
> +        ACCESS(n2rwx),
> +#undef ACCESS
> +    };
> +
> +    *ap2m = p2m_get_altp2m(v);
> +    if ( *ap2m == NULL)
> +        return 0;
> +
> +    /* Check if entry is part of the altp2m view */
> +    spin_lock(&(*ap2m)->lock);
> +    maddr = __p2m_lookup(*ap2m, gpa, NULL);
> +    spin_unlock(&(*ap2m)->lock);
> +    if ( maddr != INVALID_PADDR )
> +        return 0;
> +
> +    /* Check if entry is part of the host p2m view */
> +    spin_lock(&hp2m->lock);
> +    maddr = __p2m_lookup(hp2m, gpa, &p2mt);
> +    if ( maddr == INVALID_PADDR )
> +        goto out;
> +
> +    rc = __p2m_get_mem_access(hp2m, gfn, &xma);
> +    if ( rc )
> +        goto out;
> +
> +    rc = p2m_get_gfn_level_and_attr(hp2m, gpa, &level, &mattr);
> +    if ( rc )
> +        goto out;
Can we introduce a function which return the xma, mfn, order, attribute 
at once? It will avoid to browse the p2m 3 times which is really 
expensive on ARMv7 because the p2m is not mapped in the virtual address 
space of Xen.
> +    spin_unlock(&hp2m->lock);
> +
> +    mask = level_masks[level];
> +
> +    rc = apply_p2m_changes(d, *ap2m, INSERT,
> +                           pfn_to_paddr(gfn_x(gfn)) & mask,
> +                           (pfn_to_paddr(gfn_x(gfn)) + level_sizes[level]) & 
> mask,
> +                           maddr & mask, mattr, 0, p2mt,
> +                           memaccess[xma]);
The page associated to the MFN is not locked, so another thread could 
decide to remove the page from the domain and then the altp2m would 
contain an entry to something that does not belong to the domain 
anymore. Note that x86 is doing the same. So I am not sure why it is 
considered safe there...
> +    if ( rc )
> +    {
> +        gdprintk(XENLOG_ERR, "failed to set entry for %lx -> %lx p2m %lx\n",
> +                (unsigned long)pfn_to_paddr(gfn_x(gfn)), (unsigned 
> long)(maddr), (unsigned long)*ap2m);
> +        domain_crash(hp2m->domain);
> +    }
> +
> +    return 1;
> +
> +out:
> +    spin_unlock(&hp2m->lock);
> +    return 0;
> +}
> +
>  static void p2m_init_altp2m_helper(struct domain *d, unsigned int i)
>  {
>      struct p2m_domain *p2m = d->arch.altp2m_p2m[i];
[...]

> @@ -2429,6 +2460,8 @@ static void do_trap_data_abort_guest(struct cpu_user_regs 
> *regs,
>                                       const union hsr hsr)
>  {
>      const struct hsr_dabt dabt = hsr.dabt;
> +    struct vcpu *v = current;
> +    struct p2m_domain *p2m = NULL;
>      int rc;
>      mmio_info_t info;
>
> @@ -2449,6 +2482,12 @@ static void do_trap_data_abort_guest(struct 
> cpu_user_regs *regs,
>          info.gpa = get_faulting_ipa();
>      else
>      {
> +        /*
> +         * When using altp2m, this flush is required to get rid of old TLB
> +         * entries and use the new, lazily copied, ap2m entries.
> +         */
> +        flush_tlb_local();
Can you give more details why this flush is required?

> +
Regards,

[1] https://lists.xen.org/archives/html/xen-devel/2016-06/msg02853.html

--
Julien Grall

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