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Date: Mon, 14 Feb 2022 13:20:46 +1000
From: Nicholas Piggin <npiggin@gmail.com>
Subject: Re: [PATCH v3 06/10] powerpc/vas: Map paste address only if window is
 active
To: Haren Myneni <haren@linux.ibm.com>, linuxppc-dev@lists.ozlabs.org,
 mpe@ellerman.id.au
References: <7d175313528ea7aae20d9141f0efa2e57f44c9f4.camel@linux.ibm.com>
 <4ecd08a1b92590d6220920245c23526a68dc531c.camel@linux.ibm.com>
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Excerpts from Haren Myneni's message of January 22, 2022 5:58 am:
>=20
> The paste address mapping is done with mmap() after the window is
> opened with ioctl. But the window can be closed due to lost credit
> due to core removal before mmap(). So if the window is not active,
> return mmap() failure with -EACCES and expects the user space reissue
> mmap() when the window is active or open new window when the credit
> is available.
>=20
> Signed-off-by: Haren Myneni <haren@linux.ibm.com>
> ---
>  arch/powerpc/platforms/book3s/vas-api.c | 21 ++++++++++++++++++++-
>  1 file changed, 20 insertions(+), 1 deletion(-)
>=20
> diff --git a/arch/powerpc/platforms/book3s/vas-api.c b/arch/powerpc/platf=
orms/book3s/vas-api.c
> index a63fd48e34a7..2d06bd1b1935 100644
> --- a/arch/powerpc/platforms/book3s/vas-api.c
> +++ b/arch/powerpc/platforms/book3s/vas-api.c
> @@ -379,10 +379,27 @@ static int coproc_mmap(struct file *fp, struct vm_a=
rea_struct *vma)
>  		return -EACCES;
>  	}
> =20
> +	/*
> +	 * The initial mapping is done after the window is opened
> +	 * with ioctl. But this window might have been closed
> +	 * due to lost credit (core removal on PowerVM) before mmap().

What does "initial mapping" mean?

mapping ~=3D mmap, in kernel speak.

You will have to differentiate the concepts.

> +	 * So if the window is not active, return mmap() failure
> +	 * with -EACCES and expects the user space reconfigure (mmap)
> +	 * window when it is active again or open new window when
> +	 * the credit is available.
> +	 */
> +	mutex_lock(&txwin->task_ref.mmap_mutex);
> +	if (txwin->status !=3D VAS_WIN_ACTIVE) {
> +		pr_err("%s(): Window is not active\n", __func__);
> +		rc =3D -EACCES;
> +		goto out;
> +	}
> +
>  	paste_addr =3D cp_inst->coproc->vops->paste_addr(txwin);
>  	if (!paste_addr) {
>  		pr_err("%s(): Window paste address failed\n", __func__);
> -		return -EINVAL;
> +		rc =3D -EINVAL;
> +		goto out;
>  	}
> =20
>  	pfn =3D paste_addr >> PAGE_SHIFT;
> @@ -401,6 +418,8 @@ static int coproc_mmap(struct file *fp, struct vm_are=
a_struct *vma)
> =20
>  	txwin->task_ref.vma =3D vma;
> =20
> +out:
> +	mutex_unlock(&txwin->task_ref.mmap_mutex);

If the hypervisor can revoke a window at any point with DLPAR, it's not=20
clear *why* this is needed. The hypervisor could cause your window to=20
close right after this mmap() returns, right? So an explanation for=20
exactly what this patch is needed for beyond that would help.

Thanks,
Nick