Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
Subject: Re: [PATCH v3 02/14] KVM: s390: pv: avoid stall notifications for
 some UVCs
To:     Claudio Imbrenda <imbrenda@linux.ibm.com>, kvm@vger.kernel.org
Cc:     cohuck@redhat.com, borntraeger@de.ibm.com, frankja@linux.ibm.com,
        thuth@redhat.com, pasic@linux.ibm.com, linux-s390@vger.kernel.org,
        linux-kernel@vger.kernel.org, Ulrich.Weigand@de.ibm.com
References: <20210804154046.88552-1-imbrenda@linux.ibm.com>
 <20210804154046.88552-3-imbrenda@linux.ibm.com>
From:   David Hildenbrand <david@redhat.com>
Organization: Red Hat
Message-ID: <7ef282bb-d90f-9cbb-678b-4293790fb8c6@redhat.com>
Date:   Fri, 6 Aug 2021 09:30:04 +0200
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101
 Thunderbird/78.11.0
MIME-Version: 1.0
In-Reply-To: <20210804154046.88552-3-imbrenda@linux.ibm.com>
Content-Type: text/plain; charset=utf-8; format=flowed
Content-Language: en-US
Content-Transfer-Encoding: 7bit
Precedence: bulk

On 04.08.21 17:40, 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.
> 
> Also fix kvm_s390_pv_init_vm to avoid stalls when the system is heavily
> overcommitted.

I suggest splitting this change into a separate patch and adding a bit 
more meat to the description why using the other variant is possible in 
the called context. I was kind of surprise to find that change buried in 
this patch.

Then, you can give both patches a more descriptive patch subject.

> 
> Signed-off-by: Claudio Imbrenda <imbrenda@linux.ibm.com>
> Fixes: 214d9bbcd3a672 ("s390/mm: provide memory management functions for protected KVM guests")
> ---
>   arch/s390/kernel/uv.c | 29 +++++++++++++++++++++++------
>   arch/s390/kvm/pv.c    |  2 +-
>   2 files changed, 24 insertions(+), 7 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;
>   }

That looks conceptually like the right thing to me.

-- 
Thanks,

David / dhildenb