Quoting Tom Lendacky (2025-03-26 14:21:31)
> On 3/26/25 13:46, Tom Lendacky wrote:
> > On 3/7/25 12:15, Fabiano Rosas wrote:
> >> From: Steve Sistare <[email protected]>
> >>
> >> Unlike cpr-reboot mode, cpr-transfer mode cannot save volatile ram blocks
> >> in the migration stream file and recreate them later, because the physical
> >> memory for the blocks is pinned and registered for vfio. Add a blocker
> >> for volatile ram blocks.
> >>
> >> Also add a blocker for RAM_GUEST_MEMFD. Preserving guest_memfd may be
> >> sufficient for CPR, but it has not been tested yet.
> >>
> >> Signed-off-by: Steve Sistare <[email protected]>
> >> Reviewed-by: Fabiano Rosas <[email protected]>
> >> Reviewed-by: Peter Xu <[email protected]>
> >> Reviewed-by: David Hildenbrand <[email protected]>
> >> Message-ID: <[email protected]>
> >> Signed-off-by: Fabiano Rosas <[email protected]>
> >> ---
> >> include/exec/memory.h | 3 ++
> >> include/exec/ramblock.h | 1 +
> >> migration/savevm.c | 2 ++
> >> system/physmem.c | 66 +++++++++++++++++++++++++++++++++++++++++
> >> 4 files changed, 72 insertions(+)
> >
> > This patch breaks booting an SNP guest as it triggers the following
> > assert:
> >
> > qemu-system-x86_64: ../util/error.c:68: error_setv: Assertion `*errp ==
> > NULL' failed.
> >
> > I tracked it to the err_setg() call in ram_block_add_cpr_blocker().
> > It looks like the error message is unable to be printed because
> > rb->cpr_blocker is NULL.
> >
> > Adding aux-ram-share=on to the -machine object gets me past the error and
> > therefore the assertion, but isn't that an incompatible change to how an
> > SNP guest has to be started?
>
> If I update the err_setg() call to use the errp parameter that is passed
> into ram_block_add_cpr_blocker(), I get the following message and then
> the guest launch terminates:
>
> qemu-system-x86_64: Memory region pc.bios is not compatible with CPR.
> share=on is required for memory-backend objects, and aux-ram-share=on is
> required.
>
> The qemu parameters I used prior to this patch that allowed an SNP guest
> to launch were:
>
> -machine type=q35,confidential-guest-support=sev0,memory-backend=ram1
> -object memory-backend-memfd,id=ram1,size=16G,share=true,prealloc=false
>
> With these parameters after this patch, the launch fails.
I think it might be failing because the caller of
ram_block_add_cpr_blocker() is passing in &error_abort, but if the
error_setg() is call on a properly initialized cpr_blocker value then
SNP is still able to boot for me. I'm not sure where the best spot is
to initialize cpr_blocker, it probably needs to be done before either
ram_block_add_cpr_blocker() or ram_block_del_cpr_blocker() are callable,
but the following avoids the reported crash at least:
diff --git a/system/physmem.c b/system/physmem.c
index 44dd129662..bff0fdcaac 100644
--- a/system/physmem.c
+++ b/system/physmem.c
@@ -4176,6 +4176,7 @@ void ram_block_add_cpr_blocker(RAMBlock *rb, Error **errp)
return;
}
+ rb->cpr_blocker = NULL;
error_setg(&rb->cpr_blocker,
"Memory region %s is not compatible with CPR. share=on is "
"required for memory-backend objects, and aux-ram-share=on is "
-Mike
>
> Thanks,
> Tom
>
> >
> > Thanks,
> > Tom
> >
> >>
> >> diff --git a/include/exec/memory.h b/include/exec/memory.h
> >> index 78c4e0aec8..d09af58c97 100644
> >> --- a/include/exec/memory.h
> >> +++ b/include/exec/memory.h
> >> @@ -3203,6 +3203,9 @@ bool ram_block_discard_is_disabled(void);
> >> */
> >> bool ram_block_discard_is_required(void);
> >>
> >> +void ram_block_add_cpr_blocker(RAMBlock *rb, Error **errp);
> >> +void ram_block_del_cpr_blocker(RAMBlock *rb);
> >> +
> >> #endif
> >>
> >> #endif
> >> diff --git a/include/exec/ramblock.h b/include/exec/ramblock.h
> >> index 0babd105c0..64484cd821 100644
> >> --- a/include/exec/ramblock.h
> >> +++ b/include/exec/ramblock.h
> >> @@ -39,6 +39,7 @@ struct RAMBlock {
> >> /* RCU-enabled, writes protected by the ramlist lock */
> >> QLIST_ENTRY(RAMBlock) next;
> >> QLIST_HEAD(, RAMBlockNotifier) ramblock_notifiers;
> >> + Error *cpr_blocker;
> >> int fd;
> >> uint64_t fd_offset;
> >> int guest_memfd;
> >> diff --git a/migration/savevm.c b/migration/savevm.c
> >> index 5c4fdfd95e..ce158c3512 100644
> >> --- a/migration/savevm.c
> >> +++ b/migration/savevm.c
> >> @@ -3514,12 +3514,14 @@ void vmstate_register_ram(MemoryRegion *mr,
> >> DeviceState *dev)
> >> qemu_ram_set_idstr(mr->ram_block,
> >> memory_region_name(mr), dev);
> >> qemu_ram_set_migratable(mr->ram_block);
> >> + ram_block_add_cpr_blocker(mr->ram_block, &error_fatal);
> >> }
> >>
> >> void vmstate_unregister_ram(MemoryRegion *mr, DeviceState *dev)
> >> {
> >> qemu_ram_unset_idstr(mr->ram_block);
> >> qemu_ram_unset_migratable(mr->ram_block);
> >> + ram_block_del_cpr_blocker(mr->ram_block);
> >> }
> >>
> >> void vmstate_register_ram_global(MemoryRegion *mr)
> >> diff --git a/system/physmem.c b/system/physmem.c
> >> index 8c1736f84e..445981a1b4 100644
> >> --- a/system/physmem.c
> >> +++ b/system/physmem.c
> >> @@ -70,7 +70,10 @@
> >>
> >> #include "qemu/pmem.h"
> >>
> >> +#include "qapi/qapi-types-migration.h"
> >> +#include "migration/blocker.h"
> >> #include "migration/cpr.h"
> >> +#include "migration/options.h"
> >> #include "migration/vmstate.h"
> >>
> >> #include "qemu/range.h"
> >> @@ -1903,6 +1906,14 @@ static void ram_block_add(RAMBlock *new_block,
> >> Error **errp)
> >> qemu_mutex_unlock_ramlist();
> >> goto out_free;
> >> }
> >> +
> >> + error_setg(&new_block->cpr_blocker,
> >> + "Memory region %s uses guest_memfd, "
> >> + "which is not supported with CPR.",
> >> + memory_region_name(new_block->mr));
> >> + migrate_add_blocker_modes(&new_block->cpr_blocker, errp,
> >> + MIG_MODE_CPR_TRANSFER,
> >> + -1);
> >> }
> >>
> >> ram_size = (new_block->offset + new_block->max_length) >>
> >> TARGET_PAGE_BITS;
> >> @@ -4094,3 +4105,58 @@ bool ram_block_discard_is_required(void)
> >> return qatomic_read(&ram_block_discard_required_cnt) ||
> >> qatomic_read(&ram_block_coordinated_discard_required_cnt);
> >> }
> >> +
> >> +/*
> >> + * Return true if ram is compatible with CPR. Do not exclude rom,
> >> + * because the rom file could change in new QEMU.
> >> + */
> >> +static bool ram_is_cpr_compatible(RAMBlock *rb)
> >> +{
> >> + MemoryRegion *mr = rb->mr;
> >> +
> >> + if (!mr || !memory_region_is_ram(mr)) {
> >> + return true;
> >> + }
> >> +
> >> + /* Ram device is remapped in new QEMU */
> >> + if (memory_region_is_ram_device(mr)) {
> >> + return true;
> >> + }
> >> +
> >> + /*
> >> + * A file descriptor is passed to new QEMU and remapped, or its
> >> backing
> >> + * file is reopened and mapped. It must be shared to avoid COW.
> >> + */
> >> + if (rb->fd >= 0 && qemu_ram_is_shared(rb)) {
> >> + return true;
> >> + }
> >> +
> >> + return false;
> >> +}
> >> +
> >> +/*
> >> + * Add a blocker for each volatile ram block. This function should only
> >> be
> >> + * called after we know that the block is migratable. Non-migratable
> >> blocks
> >> + * are either re-created in new QEMU, or are handled specially, or are
> >> covered
> >> + * by a device-level CPR blocker.
> >> + */
> >> +void ram_block_add_cpr_blocker(RAMBlock *rb, Error **errp)
> >> +{
> >> + assert(qemu_ram_is_migratable(rb));
> >> +
> >> + if (ram_is_cpr_compatible(rb)) {
> >> + return;
> >> + }
> >> +
> >> + error_setg(&rb->cpr_blocker,
> >> + "Memory region %s is not compatible with CPR. share=on is "
> >> + "required for memory-backend objects, and aux-ram-share=on
> >> is "
> >> + "required.", memory_region_name(rb->mr));
> >> + migrate_add_blocker_modes(&rb->cpr_blocker, errp,
> >> MIG_MODE_CPR_TRANSFER,
> >> + -1);
> >> +}
> >> +
> >> +void ram_block_del_cpr_blocker(RAMBlock *rb)
> >> +{
> >> + migrate_del_blocker(&rb->cpr_blocker);
> >> +}
>
