Re: [PATCH v5 04/14] KVM: s390: pv: avoid stalls when making pages secure

[Janosch Frank <frankja@linux.ibm.com>]

On 9/20/21 15:24, Claudio Imbrenda wrote:
> 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).
> 
> Signed-off-by: Claudio Imbrenda <imbrenda@linux.ibm.com>
> Fixes: 214d9bbcd3a672 ("s390/mm: provide memory management functions for protected KVM guests")

Reviewed-by: Janosch Frank <frankja@linux.ibm.com>

> ---
>   arch/s390/kernel/uv.c     | 29 +++++++++++++++++++++++------
>   arch/s390/kvm/intercept.c |  5 +++++
>   2 files changed, 28 insertions(+), 6 deletions(-)
> 
> diff --git a/arch/s390/kernel/uv.c b/arch/s390/kernel/uv.c
> index aeb0a15bcbb7..68a8fbafcb9c 100644
> --- a/arch/s390/kernel/uv.c
> +++ b/arch/s390/kernel/uv.c
> @@ -180,7 +180,7 @@ 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;
> @@ -196,12 +196,25 @@ 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;
>   }
>   
>   /*
> @@ -254,6 +267,10 @@ int gmap_make_secure(struct gmap *gmap, unsigned long gaddr, void *uvcb)
>   	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) {
>   		/*
> diff --git a/arch/s390/kvm/intercept.c b/arch/s390/kvm/intercept.c
> index 72b25b7cc6ae..47833ade4da5 100644
> --- a/arch/s390/kvm/intercept.c
> +++ b/arch/s390/kvm/intercept.c
> @@ -516,6 +516,11 @@ 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;
>   }
>   
>