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KVM: s390: pv: avoid stalls when making pages secure

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Improve make_secure_pte to avoid stalls when the system is heavily
overcommitted. This was especially problematic in kvm_s390_pv_unpack,
because of the loop over all pages that needed unpacking.

Due to the locks being held, it was not possible to simply replace
uv_call with uv_call_sched. A more complex approach was
needed, in which uv_call is replaced with __uv_call, which does not
loop. When the UVC needs to be executed again, -EAGAIN is returned, and
the caller (or its caller) will try again.

When -EAGAIN is returned, the path is the same as when the page is in
writeback (and the writeback check is also performed, which is
harmless).

Fixes: 214d9bbcd3 ("s390/mm: provide memory management functions for protected KVM guests")
Signed-off-by: Claudio Imbrenda <imbrenda@linux.ibm.com>
Reviewed-by: Janosch Frank <frankja@linux.ibm.com>
Reviewed-by: Christian Borntraeger <borntraeger@de.ibm.com>
Link: https://lore.kernel.org/r/20210920132502.36111-5-imbrenda@linux.ibm.com
Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
This commit is contained in:
Claudio Imbrenda 2021-09-20 15:24:52 +02:00 committed by Christian Borntraeger
parent 1e2aa46de5
commit f0a1a0615a
2 changed files with 28 additions and 6 deletions
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29
arch/s390/kernel/uv.c
View File

@ -165,7 +165,7 @@ static int make_secure_pte(pte_t *ptep, unsigned long addr,
{
pte_t entry = READ_ONCE(*ptep);
struct page *page;
int expected, rc = 0;
int expected, cc = 0;
if (!pte_present(entry))
return -ENXIO;
@ -181,12 +181,25 @@ static int make_secure_pte(pte_t *ptep, unsigned long addr,
if (!page_ref_freeze(page, expected))
return -EBUSY;
set_bit(PG_arch_1, &page->flags);
rc = uv_call(0, (u64)uvcb);
/*
* If the UVC does not succeed or fail immediately, we don't want to
* loop for long, or we might get stall notifications.
* On the other hand, this is a complex scenario and we are holding a lot of
* locks, so we can't easily sleep and reschedule. We try only once,
* and if the UVC returned busy or partial completion, we return
* -EAGAIN and we let the callers deal with it.
*/
cc = __uv_call(0, (u64)uvcb);
page_ref_unfreeze(page, expected);
/* Return -ENXIO if the page was not mapped, -EINVAL otherwise */
if (rc)
rc = uvcb->rc == 0x10a ? -ENXIO : -EINVAL;
return rc;
/*
* Return -ENXIO if the page was not mapped, -EINVAL for other errors.
* If busy or partially completed, return -EAGAIN.
*/
if (cc == UVC_CC_OK)
return 0;
else if (cc == UVC_CC_BUSY || cc == UVC_CC_PARTIAL)
return -EAGAIN;
return uvcb->rc == 0x10a ? -ENXIO : -EINVAL;
}
/*
@ -239,6 +252,10 @@ out:
mmap_read_unlock(gmap->mm);
if (rc == -EAGAIN) {
/*
* If we are here because the UVC returned busy or partial
* completion, this is just a useless check, but it is safe.
*/
wait_on_page_writeback(page);
} else if (rc == -EBUSY) {
/*

5
arch/s390/kvm/intercept.c
View File

@ -518,6 +518,11 @@ static int handle_pv_uvc(struct kvm_vcpu *vcpu)
*/
if (rc == -EINVAL)
return 0;
/*
* If we got -EAGAIN here, we simply return it. It will eventually
* get propagated all the way to userspace, which should then try
* again.
*/
return rc;
}