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<303354146cb96d229d4ee0c148be56caf317dee2.camel@linux.ibm.com> Subject: Re: [PATCH v5 2/3] vfio/ism: Implement vfio_pci driver for ISM devices From: Niklas Schnelle To: Julian Ruess , wintera@linux.ibm.com, ts@linux.ibm.com, oberpar@linux.ibm.com, gbayer@linux.ibm.com, Alex Williamson , Jason Gunthorpe , Yishai Hadas , Shameer Kolothum , Kevin Tian Cc: mjrosato@linux.ibm.com, alifm@linux.ibm.com, raspl@linux.ibm.com, hca@linux.ibm.com, agordeev@linux.ibm.com, gor@linux.ibm.com, kvm@vger.kernel.org, linux-kernel@vger.kernel.org, linux-s390@vger.kernel.org, linux-pci@vger.kernel.org In-Reply-To: <20260317-vfio_pci_ism-v5-2-b73248b4e576@linux.ibm.com> References: <20260317-vfio_pci_ism-v5-0-b73248b4e576@linux.ibm.com> <20260317-vfio_pci_ism-v5-2-b73248b4e576@linux.ibm.com> Autocrypt: addr=schnelle@linux.ibm.com; prefer-encrypt=mutual; keydata=mQINBGHm3M8BEAC+MIQkfoPIAKdjjk84OSQ8erd2OICj98+GdhMQpIjHXn/RJdCZLa58k /ay5x0xIHkWzx1JJOm4Lki7WEzRbYDexQEJP0xUia0U+4Yg7PJL4Dg/W4Ho28dRBROoJjgJSLSHwc 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authtc= authcc= route=outbound adjust=0 reason=mlx scancount=1 engine=8.22.0-2603050001 definitions=main-2603180125 On Tue, 2026-03-17 at 13:58 +0100, Julian Ruess wrote: > Add a vfio_pci variant driver for the s390-specific Internal Shared > Memory (ISM) devices used for inter-VM communication. >=20 > This enables the development of vfio-pci-based user space drivers for > ISM devices. >=20 > On s390, kernel primitives such as ioread() and iowrite() are switched > over from function handle based PCI load/stores instructions to PCI > memory-I/O (MIO) loads/stores when these are available and not > explicitly disabled. Since these instructions cannot be used with ISM > devices, ensure that classic function handle-based PCI instructions are > used instead. >=20 > The driver is still required even when MIO instructions are disabled, as > the ISM device relies on the PCI store block (PCISTB) instruction to > perform write operations. >=20 > Stores are not fragmented, therefore one ioctl corresponds to exactly > one PCISTB instruction. User space must ensure to not write more than > 4096 bytes at once to an ISM BAR which is the maximum payload of the > PCISTB instruction. >=20 > Signed-off-by: Julian Ruess > --- --- snip --- > +/* > + * Use __zpci_load() to bypass automatic use of > + * PCI MIO instructions which are not supported on ISM devices > + */ > +#define ISM_READ(size) = \ > + static int ism_read##size(struct zpci_dev *zdev, int bar, \ > + ssize_t *filled, char __user *buf, \ > + loff_t off) \ > + { \ > + u64 req, tmp; \ > + u##size val; \ > + int ret; \ > + \ > + req =3D ZPCI_CREATE_REQ(READ_ONCE(zdev->fh), bar, sizeof(val)); \ > + ret =3D __zpci_load(&tmp, req, off); \ > + val =3D (u##size)tmp; \ > + if (copy_to_user(buf, &val, sizeof(val))) \ > + return -EFAULT; \ This was actually noticed by sashiko.dev but since not everyone knows that tool yet and it still needs filtering, I'm repeating it here. This leaks kernel stack memory when __zpci_load() fails since the copy_to_user() still happens. In v4 on the other hand you had a proper early return. > + *filled =3D sizeof(val); \ > + return ret; \ > + } > + > +ISM_READ(64); > +ISM_READ(32); > +ISM_READ(16); > +ISM_READ(8); --- snip ---