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Wed, 3 Jun 2026 12:16:01 +0000 (GMT) Message-ID: <1100af0f55ba592e37b799f7e285345410fe47a8.camel@linux.ibm.com> Subject: Re: [PATCH v2 3/3] s390/pci: Fence FMB enable/disable via sysfs for passthrough devices From: Niklas Schnelle To: Omar Elghoul , linux-s390@vger.kernel.org, linux-kernel@vger.kernel.org, kvm@vger.kernel.org Cc: hca@linux.ibm.com, gor@linux.ibm.com, agordeev@linux.ibm.com, borntraeger@linux.ibm.com, svens@linux.ibm.com, mjrosato@linux.ibm.com, alifm@linux.ibm.com, farman@linux.ibm.com, gbayer@linux.ibm.com, alex@shazbot.org In-Reply-To: <20260519224204.19154-4-oelghoul@linux.ibm.com> References: <20260519224204.19154-1-oelghoul@linux.ibm.com> <20260519224204.19154-4-oelghoul@linux.ibm.com> Autocrypt: addr=schnelle@linux.ibm.com; prefer-encrypt=mutual; keydata=mQINBGHm3M8BEAC+MIQkfoPIAKdjjk84OSQ8erd2OICj98+GdhMQpIjHXn/RJdCZLa58k /ay5x0xIHkWzx1JJOm4Lki7WEzRbYDexQEJP0xUia0U+4Yg7PJL4Dg/W4Ho28dRBROoJjgJSLSHwc 3/1pjpNlSaX/qg3ZM8+/EiSGc7uEPklLYu3gRGxcWV/944HdUyLcnjrZwCn2+gg9ncVJjsimS0ro/ 2wU2RPE4ju6NMBn5Go26sAj1owdYQQv9t0d71CmZS9Bh+2+cLjC7HvyTHKFxVGOznUL+j1a45VrVS XQ+nhTVjvgvXR84z10bOvLiwxJZ/00pwNi7uCdSYnZFLQ4S/JGMs4lhOiCGJhJ/9FR7JVw/1t1G9a UlqVp23AXwzbcoV2fxyE/CsVpHcyOWGDahGLcH7QeitN6cjltf9ymw2spBzpRnfFn80nVxgSYVG1d w75ksBAuQ/3e+oTQk4GAa2ShoNVsvR9GYn7rnsDN5pVILDhdPO3J2PGIXa5ipQnvwb3EHvPXyzakY tK50fBUPKk3XnkRwRYEbbPEB7YT+ccF/HioCryqDPWUivXF8qf6Jw5T1mhwukUV1i+QyJzJxGPh19 /N2/GK7/yS5wrt0Lwxzevc5g+jX8RyjzywOZGHTVu9KIQiG8Pqx33UxZvykjaqTMjo7kaAdGEkrHZ dVHqoPZwhCsgQARAQABtChOaWtsYXMgU2NobmVsbGUgPHNjaG5lbGxlQGxpbnV4LmlibS5jb20+iQ JXBBMBCABBAhsBBQsJCAcCBhUKCQgLAgQWAgMBAh4BAheAAhkBFiEEnbAAstJ1IDCl9y3cr+Q/Fej CYJAFAmmAWs8FCQl6sYAACgkQr+Q/FejCYJAn2g//UKzlXOgizdk0wudLooRbGzDo23ktGSPK5Oj9 9o5z6v4Jz5+qOHo5835683cqkMLM9//udA1ZcKV88LVwyfmoHChPW24cWBmOEy7RJOWCR4WeEINaO pZUGF5YOx7oKTkPs511ky2FR0Heg35754pgTuTMEpYzRXr5pNMPS8mHXcXSARFPDPaCF+uBJ9BafO L7XbpSwKRttePsWAlPHbSbloeDApBfHUhcF/pbuM9GNs+c/8V9NK+SwwqNK214t7jaSq9k+19/hfE jvU45nbiYQM4VqGCelxVFRWol93JnwPFp/JaMgxgV1VYFH9Ijtgh+qNVVBqO8bbTjioFKy1bHdprN 9GyPLDxoaI/lBg+5CwKewzazUjFd0xaqZbTXSgNK4ev/IuNI3qZV8tpvZZWwIgZU1K0Bhplt8Sku+ O9Yl2H54erq9zuzwXjqBJtoW0+MaKbe+1gZ/v2/AVE2VeQMugPUWDg+2bpJaApRkeA4xQ9XfeW6Bp It7xYrwwbVhQtWRC0sRh+QNlU9HI28wPSnLWn7HFBeWupaIrxSp4IEL3eHUn8xv4aA8lpdNsHXD/X vqOSUwy5jlTPTlemvwaC9mNHagNdVXng8C6+hxiDLhZ6xH2P4qNHTKmjW61NsdF6Y/HfWP+lmbi8/ 474UNCltDt/fP01ajqogfWZKFymoH0O0KU5pa2xhcyBTY2huZWxsZSA8bmlrbGFzLnNjaG5lbGxlQ GlibS5jb20+iQJUBBMBCAA+AhsBBQsJCAcCBhUKCQgLAgQWAgMBAh4BAheAFiEEnbAAstJ1IDCl9y 3cr+Q/FejCYJAFAmmAWusFCQl6sYAACgkQr+Q/FejCYJAtIw//WmQW/Z+SLdfrlDH5J2bvixzFNnO TOvp8uM8vcNZsxZwPXem4AeCXHayCqipxpa0iXWufEIvdMxkBxWvvM//V+rTUgQnJe6nhDxfLGklx 5Mb2H+K/ndS73ElCuA30MPYq7mHr8i3gEmi2ZFX1W47JecJ8hno/DQxhHRG7bd+GFsiKCbsjLWXNq s/VaAK9uyOTQx7m6/2nR8L+Mvl1BrRXwkj7Qp0qxfQSd4r+IVNBzNFOcrGagBqsyHrN7Is7IICktH 9VFl/G8P+hfviHQLnlxw9ltzpM1Dy6N1+BM3kbqD59gX+L6wqiLJI42eh+SHCiy35FvD3AFlYx4jZ MWE6qIgFnbwcL1kvcA7nnwfr3ZizCYPm8e334xXxslXBoRGsvjXSbAeAyZo2dvJXffNHdcDdUbJSl CfOixNGGKiQvs00X9ekfq9WmmRFvmYHu/m3lg1OXnMjFFIO41O51ZdhbEYJiqZEki7jA8Hd9xuWwQ nFDHhacU3xxivZ4BKQGQc+4XZ3yp/q6+7ux9prepRy/LeRyoaAmE67oxEsAgj+qyA3Tfy5nRTDdRQ E//gpaIt9H1VEx+68dRWHroxBQeozpnFPi25AlX3k4/EtVZjcItPWgE9iru1qT4DH3BBrz7Kd1zUw NnQC77zDJyZD2WUj1E+5bftO0aeE+7HZXj3tM/ea0K05pa2xhcyBTY2huZWxsZSA8bmlrbGFzLnNj aG5lbGxlQGdtYWlsLmNvbT6JAlQEEwEIAD4CGwEFCwkIBwIGFQoJCAsCBBYCAwECHgECF4AWIQSds ACy0nUgMKX3Ldyv5D8V6MJgkAUCaYBa6wUJCXqxgAAKCRCv5D8V6MJgkF/TEACOY2kL4NGFIbWeM5 TUhatxqe8c3RT6jvNjq32CkvaK/cSZzBkS0smddyOzxt2WnsvMgkr9cM7P+CevoMwhT3e0lgQbqBD /vXZJjWKddC+iKXeqWkjMVcgCOsWNZ7PWEzRUT5X1AEFq2zzxQAQ/bCWEYNqIbHN4b6G1Wk+2Y598 +KypZ3FS0bwiItnPQOWzOOqJCGxDxaEUuXFx4ah8HtVdtIev8jPS/5uzQO9iG2vZQUWeMEYZtfMHW sbFWqo2A3lxB+KPzNIYFhul4Lyx1CwvKUAGSHOx7FZuc2xI5DYt/Wdh2QyKFYr7xVzv3uwJjeS1+3 6gvyB7DJaQuY+PziNPv4GPr5wy0cRkJ6Ps15fgC6y6wNwoNdNXKlwiuclIsBzJKa7A0pZMIfpCpIJ bEHP7oy3drBRAhIrBx7Lx1lyqqodDqc+ok5IQ5WcKG/TOrH732mTmJX6fxYTiCVxcU4WLJSNZbrZ/ pjF0AWXs7E+onAkQy6RLg/XU1iiU5QdMvug+fTA6TpPSUMdujWtGWUt3/4nC+69AVc8tXtRQTZ7gP t7uIcQFwPqUuJGS26vl0w/6dIABQAyU9acvE3adCZra+/PBKFZi/yxT1WgV1T2mexKSWwQgLcR57J Yp5oWnQRgi/S6fAoskIWkp9UVcfAQPY0p45NwO5cZR9/g06JZmyrQhTmlrbGFzIFNjaG5lbGxlIDx uaWtzQGtlcm5lbC5vcmc+iQJUBBMBCAA+AhsBBQsJCAcCBhUKCQgLAgQWAgMBAh4BAheAFiEEnbAA stJ1IDCl9y3cr+Q/FejCYJAFAmmAWusFCQl6sYAACgkQr+Q/FejCYJAz4A/9F+dMhzu7YonagL4qh WDz5IpRD4vzYKOBZ+qwYp1ugJz1BIUppN9i68HKoS4ARfgP97Sv9GpOy9g7L0lymH2MPF8hRPK0Yn 7DKIkeu/r28YWEoWfoVm5reC+gpxMgmxBz4JScE4f6xfa7+Nw0bbTDl+nxftJD7lf/dTiruNJsXph HQnZ5wPXmxeH6XVJikfpyrGe8iJZALbtHtjlx6Omu7NvRGikenB8trrWS5W0F60ZdbqH1HdmDDcrZ pDq6LtAARHK5tGRm0SK6sZpKe3nULFeeCt7T/edk2FC6KVh4sL1jw1kyceX4DjiMffqYBPrhK5gz5 cDIixLBF9C6Wt1ObvuDBrIQf1/3q6EZrUrUuf6qtaXDMuC6cSlShm47qaPEvVYh67O9JZQ7vzvaea UI74DJUb8Pjnz7mTOmMOzsS1gUhCue4n2YSSM6ythioCGb/3bgMGTpuer3JhvZG5s5uKD9yyj8s8x 35qJkCFfjmjVx9s3vSUS48X+cUpYcMispErKzFu7C0YgKoxvJ4XTfXlDBiMFMPYcN67hsb2jeYHVJ wzE+fIZiDx9JLh1oQW2krwjweisE+3glOaKXZKi0fBtkxyH41iemLtLNYZRJopv6ykdl3hiI+Nh+a 3FZJPTo/OpqchMm8XIeDxC4NFFiPMpyLeYzIxO7eZpiGrAjVTE= Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable Date: Wed, 03 Jun 2026 14:15:01 +0200 Precedence: bulk X-Mailing-List: linux-s390@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Evolution 3.58.3 (3.58.3-1.fc43) X-TM-AS-GCONF: 00 X-Proofpoint-GUID: 68s4CpO-8DV0COXTWfyby1oqQVH0w1Ec X-Proofpoint-Spam-Details-Enc: AW1haW4tMjYwNjAzMDExNiBTYWx0ZWRfX4MGM9Ai0/HfH EE8827D5JfWn5NotZ7lDSm3fFNlhm13nbfI8a7CAMGiBKnKfr1B1qMfV80uDGX6PHdoif3ehtEb e6apq1+pvZpKDg5QyUdCREmWPTtov93ZRoACHBr1soFCu5Z6sE7kbyeJYAxVByLBaAb7HTGKIf6 zERyY4EoKrkbmj/sPVlGUqEdVRfzfY9gS2xbWormwQagDMYW3TdheHAE86eea05tX6e18SGA5mT Y83IRJQ5SsANGizF5jFj9GyMg42tBhmTcwlkjabqNca53M/8lt0tvf3kMBVhADs1jWGaeCQQR4p 06CbDuIS8GnfQrksWfyIyRlmePUlEzAfM2/LlfkgH+pIuGTfLiZN/JxG5cyG3T2YrsKmNCQtqO9 wpLoxxB4/NpTYBHm4124MxhZgEHzHrJ2E6uh9F5CxndH+7YQaxf8oRKLD0wb+Y4UjT8rAn1gmAy vt2Ea/FpzMs0hTTGsBA== X-Proofpoint-ORIG-GUID: 68s4CpO-8DV0COXTWfyby1oqQVH0w1Ec X-Authority-Analysis: v=2.4 cv=Vf3H+lp9 c=1 sm=1 tr=0 ts=6a201b06 cx=c_pps a=aDMHemPKRhS1OARIsFnwRA==:117 a=aDMHemPKRhS1OARIsFnwRA==:17 a=IkcTkHD0fZMA:10 a=FelO9ux0wxsA:10 a=VkNPw1HP01LnGYTKEx00:22 a=RnoormkPH1_aCDwRdu11:22 a=U7nrCbtTmkRpXpFmAIza:22 a=VnNF1IyMAAAA:8 a=aG-DwRxRKmWVmwbMMq4A:9 a=QEXdDO2ut3YA:10 X-Proofpoint-Virus-Version: vendor=baseguard engine=ICAP:2.0.293,Aquarius:18.0.1143,Hydra:6.1.125,FMLib:17.12.100.49 definitions=2026-06-03_04,2026-05-28_03,2025-10-01_01 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 priorityscore=1501 suspectscore=0 adultscore=0 bulkscore=0 impostorscore=0 phishscore=0 lowpriorityscore=0 malwarescore=0 clxscore=1015 spamscore=0 classifier=typeunknown authscore=0 authtc= authcc= route=outbound adjust=0 reason=mlx scancount=1 engine=8.22.0-2605210000 definitions=main-2606030116 On Tue, 2026-05-19 at 18:42 -0400, Omar Elghoul wrote: > Introduce a fence over enabling or disabling FMB via sysfs when the zPCI > device is associated with a KVM. This will allow a KVM guest to use FMB > passthrough and avoid the edge-case where the host disables FMB while the > guest is still using it, which may cause partial counter resets and > inconsistent reads which have no parallel in the architecture. >=20 > With this patch, the userspace driver, likely QEMU, is still able to enab= le > or disable the FMB using the VFIO device feature introduced in the previo= us > patch, effectively securing what is associated with the VM state and > isolating it from other processes on the host. >=20 > For VFIO devices that are not associated with a KVM (i.e., for userspace > drivers other than QEMU), this fence does not take effect. >=20 > Signed-off-by: Omar Elghoul > --- > arch/s390/pci/pci_debug.c | 11 ++++++++++- > 1 file changed, 10 insertions(+), 1 deletion(-) >=20 > diff --git a/arch/s390/pci/pci_debug.c b/arch/s390/pci/pci_debug.c > index c7ed7bf254b5..2601614b919b 100644 > --- a/arch/s390/pci/pci_debug.c > +++ b/arch/s390/pci/pci_debug.c > @@ -149,9 +149,15 @@ static ssize_t pci_perf_seq_write(struct file *file,= const char __user *ubuf, > if (!zdev) > return 0; > =20 > + mutex_lock(&zdev->kzdev_lock); > + if (zdev->kzdev) { > + rc =3D -EPERM; > + goto release_kzdev_and_out; Nit: "release" to me sounds misleading here since it's not about any of the things called "release" in the kernel but about unlocking a mutex, I'd probably go with "out_unlock_kzdev" for the label. > + } > + > rc =3D kstrtoul_from_user(ubuf, count, 10, &val); > if (rc) > - return rc; > + goto release_kzdev_and_out; > =20 > mutex_lock(&zdev->fmb_lock); > switch (val) { > @@ -163,6 +169,9 @@ static ssize_t pci_perf_seq_write(struct file *file, = const char __user *ubuf, > break; > } > mutex_unlock(&zdev->fmb_lock); > + > +release_kzdev_and_out: > + mutex_unlock(&zdev->kzdev_lock); > return rc ? rc : count; > } > =20 Can't say I love having to fence the host from being able to disable the FMB via sysfs, but the potential inconsistencies really aren't covered by the architecture and I don't see a better way. So with the nit handled feel free to add: Reviewed-by: Niklas Schnelle Thanks, Niklas