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Tue, 04 Nov 2025 15:05:10 -0800 (PST) Message-ID: <5c651be7-eac5-4c9e-a209-6db3a06c3d2e@gmail.com> Date: Tue, 4 Nov 2025 18:05:04 -0500 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [RFC PATCH] xen: privcmd: fix ioeventfd/ioreq crashing PV domain To: =?UTF-8?B?SsO8cmdlbiBHcm/Dnw==?= , Val Packett , Stefano Stabellini , Oleksandr Tyshchenko , =?UTF-8?Q?Marek_Marczykowski-G=C3=B3recki?= Cc: xen-devel@lists.xenproject.org, linux-kernel@vger.kernel.org, Qubes Developer Mailing List References: <20251015195713.6500-1-val@invisiblethingslab.com> <5a3660c9-1b18-4d87-a1f7-efa8d68239d8@suse.com> Content-Language: en-US From: Demi Marie Obenour Autocrypt: addr=demiobenour@gmail.com; keydata= xsFNBFp+A0oBEADffj6anl9/BHhUSxGTICeVl2tob7hPDdhHNgPR4C8xlYt5q49yB+l2nipd aq+4Gk6FZfqC825TKl7eRpUjMriwle4r3R0ydSIGcy4M6eb0IcxmuPYfbWpr/si88QKgyGSV Z7GeNW1UnzTdhYHuFlk8dBSmB1fzhEYEk0RcJqg4AKoq6/3/UorR+FaSuVwT7rqzGrTlscnT DlPWgRzrQ3jssesI7sZLm82E3pJSgaUoCdCOlL7MMPCJwI8JpPlBedRpe9tfVyfu3euTPLPx wcV3L/cfWPGSL4PofBtB8NUU6QwYiQ9Hzx4xOyn67zW73/G0Q2vPPRst8LBDqlxLjbtx/WLR 6h3nBc3eyuZ+q62HS1pJ5EvUT1vjyJ1ySrqtUXWQ4XlZyoEFUfpJxJoN0A9HCxmHGVckzTRl 5FMWo8TCniHynNXsBtDQbabt7aNEOaAJdE7to0AH3T/Bvwzcp0ZJtBk0EM6YeMLtotUut7h2 Bkg1b//r6bTBswMBXVJ5H44Qf0+eKeUg7whSC9qpYOzzrm7+0r9F5u3qF8ZTx55TJc2g656C 9a1P1MYVysLvkLvS4H+crmxA/i08Tc1h+x9RRvqba4lSzZ6/Tmt60DPM5Sc4R0nSm9BBff0N m0bSNRS8InXdO1Aq3362QKX2NOwcL5YaStwODNyZUqF7izjK4QARAQABzTxEZW1pIE1hcmll IE9iZW5vdXIgKGxvdmVyIG9mIGNvZGluZykgPGRlbWlvYmVub3VyQGdtYWlsLmNvbT7CwXgE EwECACIFAlp+A0oCGwMGCwkIBwMCBhUIAgkKCwQWAgMBAh4BAheAAAoJELKItV//nCLBhr8Q AK/xrb4wyi71xII2hkFBpT59ObLN+32FQT7R3lbZRjVFjc6yMUjOb1H/hJVxx+yo5gsSj5LS 9AwggioUSrcUKldfA/PKKai2mzTlUDxTcF3vKx6iMXKA6AqwAw4B57ZEJoMM6egm57TV19kz PMc879NV2nc6+elaKl+/kbVeD3qvBuEwsTe2Do3HAAdrfUG/j9erwIk6gha/Hp9yZlCnPTX+ VK+xifQqt8RtMqS5R/S8z0msJMI/ajNU03kFjOpqrYziv6OZLJ5cuKb3bZU5aoaRQRDzkFIR 6aqtFLTohTo20QywXwRa39uFaOT/0YMpNyel0kdOszFOykTEGI2u+kja35g9TkH90kkBTG+a EWttIht0Hy6YFmwjcAxisSakBuHnHuMSOiyRQLu43ej2+mDWgItLZ48Mu0C3IG1seeQDjEYP tqvyZ6bGkf2Vj+L6wLoLLIhRZxQOedqArIk/Sb2SzQYuxN44IDRt+3ZcDqsPppoKcxSyd1Ny 2tpvjYJXlfKmOYLhTWs8nwlAlSHX/c/jz/ywwf7eSvGknToo1Y0VpRtoxMaKW1nvH0OeCSVJ itfRP7YbiRVc2aNqWPCSgtqHAuVraBRbAFLKh9d2rKFB3BmynTUpc1BQLJP8+D5oNyb8Ts4x Xd3iV/uD8JLGJfYZIR7oGWFLP4uZ3tkneDfYzsFNBFp+A0oBEAC9ynZI9LU+uJkMeEJeJyQ/ 8VFkCJQPQZEsIGzOTlPnwvVna0AS86n2Z+rK7R/usYs5iJCZ55/JISWd8xD57ue0eB47bcJv VqGlObI2DEG8TwaW0O0duRhDgzMEL4t1KdRAepIESBEA/iPpI4gfUbVEIEQuqdqQyO4GAe+M kD0Hy5JH/0qgFmbaSegNTdQg5iqYjRZ3ttiswalql1/iSyv1WYeC1OAs+2BLOAT2NEggSiVO txEfgewsQtCWi8H1SoirakIfo45Hz0tk/Ad9ZWh2PvOGt97Ka85o4TLJxgJJqGEnqcFUZnJJ riwoaRIS8N2C8/nEM53jb1sH0gYddMU3QxY7dYNLIUrRKQeNkF30dK7V6JRH7pleRlf+wQcN fRAIUrNlatj9TxwivQrKnC9aIFFHEy/0mAgtrQShcMRmMgVlRoOA5B8RTulRLCmkafvwuhs6 dCxN0GNAORIVVFxjx9Vn7OqYPgwiofZ6SbEl0hgPyWBQvE85klFLZLoj7p+joDY1XNQztmfA rnJ9x+YV4igjWImINAZSlmEcYtd+xy3Li/8oeYDAqrsnrOjb+WvGhCykJk4urBog2LNtcyCj kTs7F+WeXGUo0NDhbd3Z6AyFfqeF7uJ3D5hlpX2nI9no/ugPrrTVoVZAgrrnNz0iZG2DVx46 x913pVKHl5mlYQARAQABwsFfBBgBAgAJBQJafgNKAhsMAAoJELKItV//nCLBwNIP/AiIHE8b oIqReFQyaMzxq6lE4YZCZNj65B/nkDOvodSiwfwjjVVE2V3iEzxMHbgyTCGA67+Bo/d5aQGj gn0TPtsGzelyQHipaUzEyrsceUGWYoKXYyVWKEfyh0cDfnd9diAm3VeNqchtcMpoehETH8fr RHnJdBcjf112PzQSdKC6kqU0Q196c4Vp5HDOQfNiDnTf7gZSj0BraHOByy9LEDCLhQiCmr+2 E0rW4tBtDAn2HkT9uf32ZGqJCn1O+2uVfFhGu6vPE5qkqrbSE8TG+03H8ecU2q50zgHWPdHM OBvy3EhzfAh2VmOSTcRK+tSUe/u3wdLRDPwv/DTzGI36Kgky9MsDC5gpIwNbOJP2G/q1wT1o Gkw4IXfWv2ufWiXqJ+k7HEi2N1sree7Dy9KBCqb+ca1vFhYPDJfhP75I/VnzHVssZ/rYZ9+5 1yDoUABoNdJNSGUYl+Yh9Pw9pE3Kt4EFzUlFZWbE4xKL/NPno+z4J9aWemLLszcYz/u3XnbO vUSQHSrmfOzX3cV4yfmjM5lewgSstoxGyTx2M8enslgdXhPthZlDnTnOT+C+OTsh8+m5tos8 HQjaPM01MKBiAqdPgksm1wu2DrrwUi6ChRVTUBcj6+/9IJ81H2P2gJk3Ls3AVIxIffLoY34E +MYSfkEjBz0E8CLOcAw7JIwAaeBT In-Reply-To: <5a3660c9-1b18-4d87-a1f7-efa8d68239d8@suse.com> Content-Type: multipart/signed; micalg=pgp-sha256; protocol="application/pgp-signature"; boundary="------------u8e58NBhg0Fe1VXmbFnCu0hr" This is an OpenPGP/MIME signed message (RFC 4880 and 3156) --------------u8e58NBhg0Fe1VXmbFnCu0hr Content-Type: multipart/mixed; boundary="------------NjOMfyOsKaD4eGFjlTFcA0cE"; protected-headers="v1" From: Demi Marie Obenour To: =?UTF-8?B?SsO8cmdlbiBHcm/Dnw==?= , Val Packett , Stefano Stabellini , Oleksandr Tyshchenko , =?UTF-8?Q?Marek_Marczykowski-G=C3=B3recki?= Cc: xen-devel@lists.xenproject.org, linux-kernel@vger.kernel.org, Qubes Developer Mailing List Message-ID: <5c651be7-eac5-4c9e-a209-6db3a06c3d2e@gmail.com> Subject: Re: [RFC PATCH] xen: privcmd: fix ioeventfd/ioreq crashing PV domain References: <20251015195713.6500-1-val@invisiblethingslab.com> <5a3660c9-1b18-4d87-a1f7-efa8d68239d8@suse.com> In-Reply-To: <5a3660c9-1b18-4d87-a1f7-efa8d68239d8@suse.com> Autocrypt-Gossip: addr=jgross@suse.com; keydata= xsBNBFOMcBYBCACgGjqjoGvbEouQZw/ToiBg9W98AlM2QHV+iNHsEs7kxWhKMjrioyspZKOB ycWxw3ie3j9uvg9EOB3aN4xiTv4qbnGiTr3oJhkB1gsb6ToJQZ8uxGq2kaV2KL9650I1SJve dYm8Of8Zd621lSmoKOwlNClALZNew72NjJLEzTalU1OdT7/i1TXkH09XSSI8mEQ/ouNcMvIJ NwQpd369y9bfIhWUiVXEK7MlRgUG6MvIj6Y3Am/BBLUVbDa4+gmzDC9ezlZkTZG2t14zWPvx XP3FAp2pkW0xqG7/377qptDmrk42GlSKN4z76ELnLxussxc7I2hx18NUcbP8+uty4bMxABEB AAHNH0p1ZXJnZW4gR3Jvc3MgPGpncm9zc0BzdXNlLmNvbT7CwHkEEwECACMFAlOMcK8CGwMH CwkIBwMCAQYVCAIJCgsEFgIDAQIeAQIXgAAKCRCw3p3WKL8TL8eZB/9G0juS/kDY9LhEXseh mE9U+iA1VsLhgDqVbsOtZ/S14LRFHczNd/Lqkn7souCSoyWsBs3/wO+OjPvxf7m+Ef+sMtr0 G5lCWEWa9wa0IXx5HRPW/ScL+e4AVUbL7rurYMfwCzco+7TfjhMEOkC+va5gzi1KrErgNRHH kg3PhlnRY0Udyqx++UYkAsN4TQuEhNN32MvN0Np3WlBJOgKcuXpIElmMM5f1BBzJSKBkW0Jc Wy3h2Wy912vHKpPV/Xv7ZwVJ27v7KcuZcErtptDevAljxJtE7aJG6WiBzm+v9EswyWxwMCIO RoVBYuiocc51872tRGywc03xaQydB+9R7BHPzsBNBFOMcBYBCADLMfoA44MwGOB9YT1V4KCy vAfd7E0BTfaAurbG+Olacciz3yd09QOmejFZC6AnoykydyvTFLAWYcSCdISMr88COmmCbJzn sHAogjexXiif6ANUUlHpjxlHCCcELmZUzomNDnEOTxZFeWMTFF9Rf2k2F0Tl4E5kmsNGgtSa aMO0rNZoOEiD/7UfPP3dfh8JCQ1VtUUsQtT1sxos8Eb/HmriJhnaTZ7Hp3jtgTVkV0ybpgFg w6WMaRkrBh17mV0z2ajjmabB7SJxcouSkR0hcpNl4oM74d2/VqoW4BxxxOD1FcNCObCELfIS auZx+XT6s+CE7Qi/c44ibBMR7hyjdzWbABEBAAHCwF8EGAECAAkFAlOMcBYCGwwACgkQsN6d 1ii/Ey9D+Af/WFr3q+bg/8v5tCknCtn92d5lyYTBNt7xgWzDZX8G6/pngzKyWfedArllp0Pn fgIXtMNV+3t8Li1Tg843EXkP7+2+CQ98MB8XvvPLYAfW8nNDV85TyVgWlldNcgdv7nn1Sq8g HwB2BHdIAkYce3hEoDQXt/mKlgEGsLpzJcnLKimtPXQQy9TxUaLBe9PInPd+Ohix0XOlY+Uk QFEx50Ki3rSDl2Zt2tnkNYKUCvTJq7jvOlaPd6d/W0tZqpyy7KVay+K4aMobDsodB3dvEAs6 ScCnh03dDAFgIq5nsB11j3KPKdVoPlfucX2c7kGNH+LUMbzqV6beIENfNexkOfxHfw== Autocrypt-Gossip: addr=marmarek@invisiblethingslab.com; keydata= xsBNBE5j9EwBCACbYHjxDrxFAY3n1x9KBFvjzkG1qFSTVBnH4vpD/5Na4sZq4uDDMUCjivrm MzbWYaivYj96BygdOiw7PWxYrhuW0b2WYOeGudZyApgFz42g458s78EciuhgfuWBlxr8dOEN /9ueVFHcvtZmDbHhMVPcQ0O7gwh0JmwkOsf7P7WAfYXsQlhO/EBRrNXR0Je+GEpYADhRktxX h1d3Iz+oKYuwHioLX8ovoAT4+peOuecWUSpUWebpDbTR5i7NRP3PIblB4KzWJa2kh/f3mx4v SRGnHn+BfX42xSe0X7Ktl4Xf+KNq9Wkcjk2CZP57hV2v4pO0ZUOXD7IhlZtnfNj67WjdABEB AAHNPU1hcmVrIE1hcmN6eWtvd3NraS1Hw7NyZWNraSA8bWFybWFyZWtAaW52aXNpYmxldGhp bmdzbGFiLmNvbT7CwHoEEwEIACQCGwMFCwkIBwMFFQoJCAsFFgIDAQACHgECF4AFAll+l7cC GQEACgkQ24/THMrX1yw6kAgAiKiUhzAPXZj5ndqiQDl8u8PUK34SupLzYNMJOCBw5Wh+CPHe XYlQUwfULWxmzjiWCzzWDx2X/ONsYdRGKDKMqG5srOSWe1IYXv00MEutGsK+m/hmC5mqi/97 DVNZ1VtKj5WW79IsI0/7ueHsQYNNrXyOfZvKsRE8VIUJ0tNfLFDFlNpq9jONuF+GviMWxrA5 FoVaGmjh63xC0fOQYqhP2v8dbYS4B6bO5NZKI2cTHb9Li2iY0e7wIoNgvqgtR3Iv2U2Ry0yL D3mNQhwyxcWChexlymjfqLEZwKqaIOo57HOpt7OA+bMg6MvkdUTjNWf2GE6fqCcALjcToJ3L NDc1KM7ATQROY/RMAQgAtRWgUZ5mOy+c/qzmiVnxqDkiOJjmnIh3Pn+OqCtjcrTyPI9eVc06 uH30Jkco0soLiG/UgwVw4XwBlm95j9n6TSUms4mPBh1YiR1hBjsjYwn8zp/Ue9xWk1N6E14H aj55GxmS2H3YIlOXfQLr0X3RHsmKixTOKyisrYlJu71FmettDFV7CgMXy1Bc1LbAE08asvAS ShHFdRiRRtkuVHvY/Ebq9L54kOxtlI6ahrflMcT0YCMON5oe4GgQRh3p2uy+d/LS2bgRcQST IebErj8x0lM271f97GvxV/ypHo7XVIDI5FX1u31Agzx3HQr035GHt4HV4/GVCz+V4xt4BonB tQARAQABwsBfBBgBAgAJBQJOY/RMAhsMAAoJENuP0xzK19cs5MgH/jWLXil2Ud4TdtWnBxc+ 2/QZZk2JCssc1PgWNzvH5wH7U+8lGSlUK8ZMOqrrF8C5rX0+xEn7deSrsZChIOnUFo8rhCZK y/mBV+FhkMj24FZZ0n8w3eF4KF2t68Pt+AvMjxQHwxAMdf3QftgQhD0qYkt/28eedUQ+jwz6 kipc4qUQmqTEViQRPa3WAnKgNDQUDUwNruzthfGvHUjllf7zbPI8gkbARM0KlTkLikc9u+Ni VMbJTiGPB7YHyw2MIPq1n+mhSPAyXE6CVBnYkonQ7P3SLZssxC3PIarV+DTU68umQB3pfrfF 7hMcAY5csWrK9/x/Zz4RUfgN6Q3HLrSp9UQ= --------------NjOMfyOsKaD4eGFjlTFcA0cE Content-Type: multipart/mixed; boundary="------------FIWwo2ISlVmpMwZYZF2bDdOu" --------------FIWwo2ISlVmpMwZYZF2bDdOu Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: quoted-printable On 11/4/25 07:15, J=C3=BCrgen Gro=C3=9F wrote: > On 15.10.25 21:57, Val Packett wrote: >> Starting a virtio backend in a PV domain would panic the kernel in >> alloc_ioreq, trying to dereference vma->vm_private_data as a pages >> pointer when in reality it stayed as PRIV_VMA_LOCKED. >> >> Fix by allocating a pages array in mmap_resource in the PV case, >> filling it with page info converted from the pfn array. This allows >> ioreq to function successfully with a backend provided by a PV dom0. >> >> Signed-off-by: Val Packett >> --- >> I've been porting the xen-vhost-frontend[1] to Qubes, which runs on am= d64 >> and we (still) use PV for dom0. The x86 part didn't give me much troub= le, >> but the first thing I found was this crash due to using a PV domain to= host >> the backend. alloc_ioreq was dereferencing the '1' constant and panick= ing >> the dom0 kernel. >> >> I figured out that I can make a pages array in the expected format fro= m the >> pfn array where the actual memory mapping happens for the PV case, and= with >> the fix, the ioreq part works: the vhost frontend replies to the probi= ng >> sequence and the guest recognizes which virtio device is being provide= d. >> >> I still have another thing to debug: the MMIO accesses from the inner = driver >> (e.g. virtio_rng) don't get through to the vhost provider (ioeventfd d= oes >> not get notified), and manually kicking the eventfd from the frontend >> seems to crash... Xen itself?? (no Linux panic on console, just a free= ze and >> quick reboot - will try to set up a serial console now) >=20 > IMHO for making the MMIO accesses work you'd need to implement ioreq-se= rver > support for PV-domains in the hypervisor. This will be a major endeavor= , so > before taking your Linux kernel patch I'd like to see this covered. Would fixing this be a good use of time, or would it be better to focus on switching to PVH dom0? I don't know if it makes sense to spend effort on PV dom0 when dom0 isn't going to be PV indefinitely. Edera might well be interested in the PV case, as they run in cloud VMs without nested virtualization. That's not relevant to Qubes OS, though. >> But I figured I'd post this as an RFC already, since the other bug may= be >> unrelated and the ioreq area itself does work now. I'd like to hear so= me >> feedback on this from people who actually know Xen :) >=20 > My main problem with your patch is that it is adding a memory allocatio= n > for a very rare use case impacting all current users of that functional= ity. >=20 > You could avoid that by using a different ioctl which could be selected= by > specifying a new flag when calling xenforeignmemory_open() (have a look= > into the Xen sources under tools/libs/foreignmemory/core.c). Should there at least be a check to prevent the kernel from crashing? I'd expect an unsupported use of the API to return an error, not cause the kernel to oops. --=20 Sincerely, Demi Marie Obenour (she/her/hers) --------------FIWwo2ISlVmpMwZYZF2bDdOu Content-Type: application/pgp-keys; name="OpenPGP_0xB288B55FFF9C22C1.asc" Content-Disposition: attachment; filename="OpenPGP_0xB288B55FFF9C22C1.asc" Content-Description: OpenPGP public key Content-Transfer-Encoding: quoted-printable -----BEGIN PGP PUBLIC KEY BLOCK----- xsFNBFp+A0oBEADffj6anl9/BHhUSxGTICeVl2tob7hPDdhHNgPR4C8xlYt5q49y B+l2nipdaq+4Gk6FZfqC825TKl7eRpUjMriwle4r3R0ydSIGcy4M6eb0IcxmuPYf bWpr/si88QKgyGSVZ7GeNW1UnzTdhYHuFlk8dBSmB1fzhEYEk0RcJqg4AKoq6/3/ UorR+FaSuVwT7rqzGrTlscnTDlPWgRzrQ3jssesI7sZLm82E3pJSgaUoCdCOlL7M MPCJwI8JpPlBedRpe9tfVyfu3euTPLPxwcV3L/cfWPGSL4PofBtB8NUU6QwYiQ9H zx4xOyn67zW73/G0Q2vPPRst8LBDqlxLjbtx/WLR6h3nBc3eyuZ+q62HS1pJ5EvU T1vjyJ1ySrqtUXWQ4XlZyoEFUfpJxJoN0A9HCxmHGVckzTRl5FMWo8TCniHynNXs BtDQbabt7aNEOaAJdE7to0AH3T/Bvwzcp0ZJtBk0EM6YeMLtotUut7h2Bkg1b//r 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