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Re: [Xen-devel] [PATCH v12 5/6] x86/ioreq server: Asynchronously reset outstanding p2m_ioreq_server entries.
On 4/7/2017 6:22 PM, George Dunlap wrote:
On 07/04/17 10:53, Yu Zhang wrote:
On 4/7/2017 5:40 PM, Jan Beulich wrote:
On 06.04.17 at 17:53, <yu.c.zhang@xxxxxxxxxxxxxxx> wrote:
--- a/xen/arch/x86/mm/p2m-ept.c
+++ b/xen/arch/x86/mm/p2m-ept.c
@@ -544,6 +544,12 @@ static int resolve_misconfig(struct p2m_domain
*p2m, unsigned long gfn)
e.ipat = ipat;
if ( e.recalc && p2m_is_changeable(e.sa_p2mt) )
{
+ if ( e.sa_p2mt == p2m_ioreq_server )
+ {
+ ASSERT(p2m->ioreq.entry_count > 0);
+ p2m->ioreq.entry_count--;
+ }
+
e.sa_p2mt = p2m_is_logdirty_range(p2m, gfn
+ i, gfn + i)
? p2m_ram_logdirty : p2m_ram_rw;
I don't think this can be right: Why would it be valid to change the
type from p2m_ioreq_server to p2m_ram_rw (or p2m_ram_logdirty)
here, without taking into account further information? This code
can run at any time, not just when you want to reset things. So at
the very least there is a check missing whether a suitable ioreq
server still exists (and only if it doesn't you want to do the type
reset).
Sorry, Jan. I think we have discussed this quite long ago.
Indeed, there's information lacked here, and that's why global_logdirty
is disallowed
when there's remaining p2m_ioreq_server entries. :-)
@@ -816,6 +822,22 @@ ept_set_entry(struct p2m_domain *p2m, unsigned
long gfn, mfn_t mfn,
new_entry.suppress_ve = is_epte_valid(&old_entry) ?
old_entry.suppress_ve : 1;
+ /*
+ * p2m_ioreq_server is only used for 4K pages, so the
+ * count shall only happen on ept page table entries.
+ */
+ if ( p2mt == p2m_ioreq_server )
+ {
+ ASSERT(i == 0);
+ p2m->ioreq.entry_count++;
+ }
+
+ if ( ept_entry->sa_p2mt == p2m_ioreq_server )
+ {
+ ASSERT(p2m->ioreq.entry_count > 0 && i == 0);
I think this would better be two ASSERT()s, so if one triggers it's
clear what problem it was right away. The two conditions aren't
really related to one another.
@@ -965,7 +987,7 @@ static mfn_t ept_get_entry(struct p2m_domain *p2m,
if ( is_epte_valid(ept_entry) )
{
if ( (recalc || ept_entry->recalc) &&
- p2m_is_changeable(ept_entry->sa_p2mt) )
+ p2m_check_changeable(ept_entry->sa_p2mt) )
I think the distinction between these two is rather arbitrary, and I
also think this is part of the problem above: Distinguishing log-dirty
from ram-rw requires auxiliary data to be consulted. The same
ought to apply to ioreq-server, and then there wouldn't be a need
to have two p2m_*_changeable() flavors.
Well, I think we have also discussed this quite long ago, here is the link.
https://lists.xen.org/archives/html/xen-devel/2016-09/msg01017.html
Of course the subsequent use p2m_is_logdirty_range() may then
need amending.
In the end it looks like you have the inverse problem here compared
to above: You should return ram-rw when the reset was already
initiated. At least that's how I would see the logic to match up with
the log-dirty handling (where the _effective_ rather than the last
stored type is being returned).
@@ -606,6 +615,8 @@ p2m_pt_set_entry(struct p2m_domain *p2m, unsigned
long gfn, mfn_t mfn,
if ( page_order == PAGE_ORDER_4K )
{
+ p2m_type_t p2mt_old;
+
rc = p2m_next_level(p2m, &table, &gfn_remainder, gfn,
L2_PAGETABLE_SHIFT - PAGE_SHIFT,
L2_PAGETABLE_ENTRIES,
PGT_l1_page_table, 1);
@@ -629,6 +640,21 @@ p2m_pt_set_entry(struct p2m_domain *p2m,
unsigned long gfn, mfn_t mfn,
if ( entry_content.l1 != 0 )
p2m_add_iommu_flags(&entry_content, 0, iommu_pte_flags);
+ p2mt_old = p2m_flags_to_type(l1e_get_flags(*p2m_entry));
+
+ /*
+ * p2m_ioreq_server is only used for 4K pages, so
+ * the count shall only be performed for level 1 entries.
+ */
+ if ( p2mt == p2m_ioreq_server )
+ p2m->ioreq.entry_count++;
+
+ if ( p2mt_old == p2m_ioreq_server )
+ {
+ ASSERT(p2m->ioreq.entry_count > 0);
+ p2m->ioreq.entry_count--;
+ }
+
/* level 1 entry */
p2m->write_p2m_entry(p2m, gfn, p2m_entry, entry_content, 1);
I think to match up with EPT you also want to add
ASSERT(p2mt_old != p2m_ioreq_server);
to the 2M and 1G paths.
Is this really necessary? 2M and 1G page does not have p2mt_old,
defining one and peek the p2m type just
to have an ASSERT does not seem quite useful - and will hurt the
performance.
As to ept, since there's already a variable 'i', which may be greater
than 0 - so I added an ASSERT.
Yes, that's Jan's point -- that for EPT, there is effectively ASSERT()
that 2M and 1G entries are not p2m_ioreq_server; but for SVM, because of
the code duplication, there is not.
ASSERT()s are:
1. There to double-check that the assumptions you're making (i.e., "2M
and 1G entries can never be of type p2m_ioreq_server") are valid
2. Only enabled when debug=y, and so are generally not a performance
consideration.
You're making an assumption, so an ASSERT is useful; and it's only a
one-line check that will be removed for non-debug builds, so the
performance is not a consideration.
Thanks George.
I do not worry about the cost of the ASSERT() itself, but the effort of
peeking a p2m:
p2m_flags_to_type(l1e_get_flags(*p2m_entry));
And this cannot be removed during runtime.
Yu
-George
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