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Re: [PATCH V3 21/23] xen/arm: Add mapcache invalidation handling
On 11/12/2020 19:07, Stefano Stabellini wrote:
On Fri, 11 Dec 2020, Oleksandr wrote:
On 11.12.20 03:28, Stefano Stabellini wrote:
On Thu, 10 Dec 2020, Julien Grall wrote:
On 10/12/2020 02:30, Stefano Stabellini wrote:
On Mon, 30 Nov 2020, Oleksandr Tyshchenko wrote:
From: Oleksandr Tyshchenko <oleksandr_tyshchenko@xxxxxxxx>
We need to send mapcache invalidation request to qemu/demu everytime
the page gets removed from a guest.
At the moment, the Arm code doesn't explicitely remove the existing
mapping before inserting the new mapping. Instead, this is done
implicitely by __p2m_set_entry().
So we need to recognize a case when old entry is a RAM page *and*
the new MFN is different in order to set the corresponding flag.
The most suitable place to do this is p2m_free_entry(), there
we can find the correct leaf type. The invalidation request
will be sent in do_trap_hypercall() later on.
Why is it sent in do_trap_hypercall() ?
I believe this is following the approach used by x86. There are actually
some
discussion about it (see [1]).
Leaving aside the toolstack case for now, AFAIK, the only way a guest can
modify its p2m is via an hypercall. Do you have an example otherwise?
OK this is a very important assumption. We should write it down for sure.
I think it is true today on ARM.
When sending the invalidation request, the vCPU will be blocked until all
the
IOREQ server have acknowledged the invalidation. So the hypercall seems to
be
the best position to do it.
Alternatively, we could use check_for_vcpu_work() to check if the mapcache
needs to be invalidated. The inconvenience is we would execute a few more
instructions in each entry/exit path.
Yeah it would be more natural to call it from check_for_vcpu_work(). If
we put it between #ifdef CONFIG_IOREQ_SERVER it wouldn't be bad. But I
am not a fan of increasing the instructions on the exit path either.
From this point of view, putting it at the end of do_trap_hypercall is a
nice trick actually. Let's just make sure it has a good comment on top.
Signed-off-by: Oleksandr Tyshchenko <oleksandr_tyshchenko@xxxxxxxx>
CC: Julien Grall <julien.grall@xxxxxxx>
---
Please note, this is a split/cleanup/hardening of Julien's PoC:
"Add support for Guest IO forwarding to a device emulator"
Changes V1 -> V2:
- new patch, some changes were derived from (+ new explanation):
xen/ioreq: Make x86's invalidate qemu mapcache handling common
- put setting of the flag into __p2m_set_entry()
- clarify the conditions when the flag should be set
- use domain_has_ioreq_server()
- update do_trap_hypercall() by adding local variable
Changes V2 -> V3:
- update patch description
- move check to p2m_free_entry()
- add a comment
- use "curr" instead of "v" in do_trap_hypercall()
---
---
xen/arch/arm/p2m.c | 24 ++++++++++++++++--------
xen/arch/arm/traps.c | 13 ++++++++++---
2 files changed, 26 insertions(+), 11 deletions(-)
diff --git a/xen/arch/arm/p2m.c b/xen/arch/arm/p2m.c
index 5b8d494..9674f6f 100644
--- a/xen/arch/arm/p2m.c
+++ b/xen/arch/arm/p2m.c
@@ -1,6 +1,7 @@
#include <xen/cpu.h>
#include <xen/domain_page.h>
#include <xen/iocap.h>
+#include <xen/ioreq.h>
#include <xen/lib.h>
#include <xen/sched.h>
#include <xen/softirq.h>
@@ -749,17 +750,24 @@ static void p2m_free_entry(struct p2m_domain
*p2m,
if ( !p2m_is_valid(entry) )
return;
- /* Nothing to do but updating the stats if the entry is a
super-page. */
- if ( p2m_is_superpage(entry, level) )
+ if ( p2m_is_superpage(entry, level) || (level == 3) )
{
- p2m->stats.mappings[level]--;
- return;
- }
+#ifdef CONFIG_IOREQ_SERVER
+ /*
+ * If this gets called (non-recursively) then either the
entry
+ * was replaced by an entry with a different base (valid
case) or
+ * the shattering of a superpage was failed (error case).
+ * So, at worst, the spurious mapcache invalidation might be
sent.
+ */
+ if ( domain_has_ioreq_server(p2m->domain) &&
+ (p2m->domain == current->domain) &&
p2m_is_ram(entry.p2m.type) )
+ p2m->domain->mapcache_invalidate = true;
Why the (p2m->domain == current->domain) check? Shouldn't we set
mapcache_invalidate to true anyway? What happens if p2m->domain !=
current->domain? We wouldn't want the domain to lose the
mapcache_invalidate notification.
This is also discussed in [1]. :) The main question is why would a
toolstack/device model modify the guest memory after boot?
If we assume it does, then the device model would need to pause the domain
before modifying the RAM.
We also need to make sure that all the IOREQ servers have invalidated
the mapcache before the domain run again.
This would require quite a bit of work. I am not sure the effort is worth
if
there are no active users today.
OK, that explains why we think p2m->domain == current->domain, but why
do we need to have a check for it right here?
In other words, we don't think it is realistc to get here with
p2m->domain != current->domain, but let's say that we do somehow. What's
the best course of action? Probably, set mapcache_invalidate to true and
possibly print a warning?
Leaving mapcache_invalidate to false doesn't seem to be what we want to
do?
BUILD_BUG_ON(NR_hypercalls < ARRAY_SIZE(arm_hypercall_table) );
@@ -1459,7 +1460,7 @@ static void do_trap_hypercall(struct
cpu_user_regs
*regs, register_t *nr,
return;
}
- current->hcall_preempted = false;
+ curr->hcall_preempted = false;
perfc_incra(hypercalls, *nr);
call = arm_hypercall_table[*nr].fn;
@@ -1472,7 +1473,7 @@ static void do_trap_hypercall(struct
cpu_user_regs
*regs, register_t *nr,
HYPERCALL_RESULT_REG(regs) = call(HYPERCALL_ARGS(regs));
#ifndef NDEBUG
- if ( !current->hcall_preempted )
+ if ( !curr->hcall_preempted )
{
/* Deliberately corrupt parameter regs used by this
hypercall.
*/
switch ( arm_hypercall_table[*nr].nr_args ) {
@@ -1489,8 +1490,14 @@ static void do_trap_hypercall(struct
cpu_user_regs
*regs, register_t *nr,
#endif
/* Ensure the hypercall trap instruction is re-executed. */
- if ( current->hcall_preempted )
+ if ( curr->hcall_preempted )
regs->pc -= 4; /* re-execute 'hvc #XEN_HYPERCALL_TAG' */
+
+#ifdef CONFIG_IOREQ_SERVER
+ if ( unlikely(curr->domain->mapcache_invalidate) &&
+ test_and_clear_bool(curr->domain->mapcache_invalidate) )
+ ioreq_signal_mapcache_invalidate();
Why not just:
if ( unlikely(test_and_clear_bool(curr->domain->mapcache_invalidate)) )
ioreq_signal_mapcache_invalidate();
This seems to match the x86 code. My guess is they tried to prevent the
cost
of the atomic operation if there is no chance mapcache_invalidate is true.
I am split whether the first check is worth it. The atomic operation
should be
uncontended most of the time, so it should be quick. But it will always be
slower than just a read because there is always a store involved.
I am not a fun of optimizations with unclear benefits :-)
On a related topic, Jan pointed out that the invalidation would not work
properly if you have multiple vCPU modifying the P2M at the same time.
Thanks to Julien, he explained all bits in detail. Indeed I followed how it
was done on x86 (place where to send the invalidation request, the code to
check whether the flag is set, which at first glance, appears odd, etc)
and review comments (to latch current into the local variable, and make sure
that domain sends invalidation request on itself).
Regarding what to do if p2m->domain != current->domain in p2m_free_entry().
Probably we could set flag only if guest is paused, otherwise just print a
warning. Thoughts?
I'd do something like:
if ( domain_has_ioreq_server(p2m->domain) && p2m_is_ram(entry.p2m.type) )
{
WARN_ON(p2m->domain != current->domain)
IOREQ server are not trusted. Yet they will be able to reach this patch
if one re-use the stubdomain model (they are allowed to modify guest
layout).
So this change would hand a DoS attack to the IOREQ server on a silver
platter :).
In general, we should avoid to use WARN_ON() for things that can be
triggered by a domain. Instead we should use gprintk(XENLOG_WARNING,
"...") to allow rate-limit.
On the cons side, it would be more difficult to spot any misue with a
gprintk().
p2m->domain->mapcache_invalidate = true;
}
but maybe Julien has a better idea.
I suggested a different approach and some rationale in answer to your
e-mail. Although, I am not sure if we could call it a better approach
:). We can continue the discusison there.
Cheers,
--
Julien Grall
|
