Re: [Xen-devel] [PATCH v10 5/6] x86/ioreq server: Asynchronously reset outstanding p2m_ioreq_server entries.

[George Dunlap <George.Dunlap@xxxxxxxxxxxxx>]

On Sun, Apr 2, 2017 at 1:24 PM, Yu Zhang <yu.c.zhang@xxxxxxxxxxxxxxx> wrote:
> After an ioreq server has unmapped, the remaining p2m_ioreq_server
> entries need to be reset back to p2m_ram_rw. This patch does this
> asynchronously with the current p2m_change_entry_type_global()
> interface.
>
> New field entry_count is introduced in struct p2m_domain, to record
> the number of p2m_ioreq_server p2m page table entries. One nature of
> these entries is that they only point to 4K sized page frames, because
> all p2m_ioreq_server entries are originated from p2m_ram_rw ones in
> p2m_change_type_one(). We do not need to worry about the counting for
> 2M/1G sized pages.

Assuming that all p2m_ioreq_server entries are *created* by
p2m_change_type_one() may valid, but can you assume that they are only
ever *removed* by p2m_change_type_one() (or recalculation)?

What happens, for instance, if a guest balloons out one of the ram
pages?  I don't immediately see anything preventing a p2m_ioreq_server
page from being ballooned out, nor anything on the
decrease_reservation() path decreasing p2m->ioreq.entry_count.  Or did
I miss something?

Other than that, only one minor comment...

> diff --git a/xen/arch/x86/mm/hap/hap.c b/xen/arch/x86/mm/hap/hap.c
> index a57b385..6ec950a 100644
> --- a/xen/arch/x86/mm/hap/hap.c
> +++ b/xen/arch/x86/mm/hap/hap.c
> @@ -187,6 +187,15 @@ out:
>   */
>  static int hap_enable_log_dirty(struct domain *d, bool_t log_global)
>  {
> +    struct p2m_domain *p2m = p2m_get_hostp2m(d);
> +
> +    /*
> +     * Refuse to turn on global log-dirty mode if
> +     * there's outstanding p2m_ioreq_server pages.

Grammar nit: if *there are* outstanding pages.

 -George

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