Re: [Xen-devel] Some problems about xenpaging

[George Dunlap <dunlapg@xxxxxxxxx>]

On Fri, Sep 23, 2011 at 10:35 AM, Hongkaixing <hongkaixing@xxxxxxxxxx> wrote:
> Hi, Olaf
>
> we have tested the xenpaging feature and found some problems.
> (1) the test case like this : when we start a VM with POD enable, the 
> xenpaging is started at the same time.
>    this case will cause many problems ,finally, we fixed the BUG, the patch 
> is attached below.
>
> (2) there is a very serious problem. we have observed many VM crash examples, 
> the error code is not always the same.
>     we guess there exists conflict between xenpaging and memory mapping.
>     for instance, if the Dom0 map the DomU's memory to its own space , then 
> the memory of DomainU is paged out, when the Domain0 access this memory area, 
> a panic is caused.
>     And I really do not know whether the qemu device can perceive the memory 
> modified by xenpaging?
>
>    thanks!
>
> here is the patch to solve the pod problem
> 1) fix the p2m_pod_decrease_reservation() function, take care of the paging 
> type
> --- ./origin/xen/arch/x86/mm/p2m.c      2011-09-05 20:39:30.000000000 +0800
> +++ ./b/xen/arch/x86/mm/p2m.c   2011-09-23 23:46:19.000000000 +0800

Kaixing,

Xie xie ni dui wo men shuo zhe xie shi.  (Dui bu qi wo de zhong wen bu
hao.)  As Olaf says, it would help us a lot if you could submit the
patches in a form that is easier for us to apply and to comment on:
namely, either having all the patches in separate files as
attachments, or having one e-mail per patch, with the patch being the
last thing in the e-mail.

There are some suggestions on how to post patches here:
http://wiki.xensource.com/xenwiki/SubmittingXenPatches.  My preferred
technique is to make a patch series using Mercurial Queues, and then
use the PatchBomb extension (mentioned in the link above) to mail them
as a series, by using "hg email -o"

 -George

> @@ -675,6 +675,23 @@
>             BUG_ON(p2md->pod.entry_count < 0);
>             pod--;
>         }
> +        else if ( steal_for_cache && p2m_is_paging(t) )
> +        {
> +            struct page_info *page;
> +            /* alloc a new page to compensate the pod list */
> +            page = alloc_domheap_page(d, 0);
> +            if ( unlikely(page == NULL) )
> +            {
> +                goto out_entry_check;
> +            }
> +            set_p2m_entry(d, gpfn + i, _mfn(INVALID_MFN), 0, p2m_invalid);
> +            p2m_mem_paging_drop_page(d, gpfn+i);
> +            p2m_pod_cache_add(d, page, 0);
> +            steal_for_cache =  ( p2md->pod.entry_count > p2md->pod.count );
> +            nonpod--;
> +            ram--;
> +        }
> +        /* for other ram types */
>         else if ( steal_for_cache && p2m_is_ram(t) )
>         {
>             struct page_info *page;
>
> 2) fix the race between POD and xenpaging
>   situation can be described as the follow
>
>   xenpaging                  POD
>
>  mfn = gfn_to_mfn()         mfn = gfn_to_mfn()
>   check p2m type           check p2mt
>                            p2m_lock
>                            change p2m type
>                            p2m_unlock
>                            add to pod list
>    p2m_lock()
>    change p2m type
>    p2m_unlock
>    put_page()
>
> the result is a page is added to the pod list,and then it's removed from the 
> list by put_page.
> my suggestion is to extend the range of the p2m lock to contain the p2m check.
>
> in  p2m_mem_paging_nominate() function
> @@ -2532,7 +2561,8 @@
>     mfn_t mfn;
>     int ret;
>
> -    mfn = gfn_to_mfn(d, gfn, &p2mt);
> +    p2m_lock(d->arch.p2m);
> +    mfn = gfn_to_mfn_query(d, gfn, &p2mt);
>
>     /* Check if mfn is valid */
>     ret = -EINVAL;
> @@ -2580,13 +2610,12 @@
>         goto out;
>
>     /* Fix p2m entry */
> -    p2m_lock(d->arch.p2m);
>     set_p2m_entry(d, gfn, mfn, 0, p2m_ram_paging_out);
> -    p2m_unlock(d->arch.p2m);
>
>     ret = 0;
>
>  out:
> +    p2m_unlock(d->arch.p2m);
>     return ret;
>  }
> in
> @@ -2595,34 +2624,39 @@
>     struct page_info *page;
>     p2m_type_t p2mt;
>     mfn_t mfn;
> -
> +    int ret;
> +    p2m_lock(d->arch.p2m);
>     /* Get mfn */
> -    mfn = gfn_to_mfn(d, gfn, &p2mt);
> +    mfn = gfn_to_mfn_query(d, gfn, &p2mt);
> +
> +    ret = -EINVAL;
>     if ( unlikely(!mfn_valid(mfn)) )
> -        return -EINVAL;
> +        goto out;
>
>     if (p2mt != p2m_ram_paging_out)
>      {
>          printk("p2m_mem_paging_evict type %d\n", p2mt);
> -               return -EINVAL;
> +         goto out;
>      }
>     /* Get the page so it doesn't get modified under Xen's feet */
>     page = mfn_to_page(mfn);
>     if ( unlikely(!get_page(page, d)) )
> -        return -EINVAL;
> +        goto out;
>
>     /* Decrement guest domain's ref count of the page */
>     if ( test_and_clear_bit(_PGC_allocated, &page->count_info) )
>         put_page(page);
>
>     /* Remove mapping from p2m table */
> -    p2m_lock(d->arch.p2m);
>     set_p2m_entry(d, gfn, _mfn(PAGING_MFN), 0, p2m_ram_paged);
> -    p2m_unlock(d->arch.p2m);
>
>     /* Put the page back so it gets freed */
>     put_page(page);
> +
> +    ret = 0;
>
> +out:
> +    p2m_unlock(d->arch.p2m);
>     return 0;
>  }
>
> 3) fix the vmx_load_pdptrs() function in  vmx.c
> in this situation the page directory table is paged out.
> Although using mdelay() is a bad idea, it's better than making the xen crash
>
> void vmx_load_pdptrs(struct vcpu *v)
> {
>    unsigned long cr3 = v->arch.hvm_vcpu.guest_cr[3], mfn;
>    uint64_t *guest_pdptrs;
>    p2m_type_t p2mt;
>    char *p;
>    unsigned int try_count = 0;
>    /* EPT needs to load PDPTRS into VMCS for PAE. */
>    if ( !hvm_pae_enabled(v) || (v->arch.hvm_vcpu.guest_efer & EFER_LMA) )
>        return;
>    if ( cr3 & 0x1fUL )
>        goto crash;
>    mfn = mfn_x(gfn_to_mfn(v->domain, cr3 >> PAGE_SHIFT, &p2mt));
>    if ( !p2m_is_ram(p2mt) )
>        goto crash;
> +  if( p2m_is_paging(p2mt))
> +  {
> +        p2m_mem_paging_populate(v->domain, cr3 >> PAGE_SHIFT);
> +        do
> +        {
> +             mdelay(1);
> +             try_count ++;
> +             if ( try_count > 65535 )
> +             {
> +                 goto crash;
> +             }
> +             mfn = mfn_x(gfn_to_mfn(v->domain, cr3 >> PAGE_SHIFT, &p2mt));
> +        }while( !mfn_valid(mfn));
> +    }
> +
>    p = map_domain_page(mfn);
>
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