On Sat, 11 Nov 2023 10:39:40 +0100 Heinrich Schuchardt xypron.glpk@gmx.de wrote:
Hi Heinrich,
thanks for having a look!
On 11/10/23 15:16, Andre Przywara wrote:
On Fri, 10 Nov 2023 05:53:59 -0700 Simon Glass sjg@chromium.org wrote:
Hi Simon,
On Tue, 7 Nov 2023 at 09:09, Andre Przywara andre.przywara@arm.com wrote:
According to the virtio v1.x "entropy device" specification, a virtio-rng device is supposed to always return at least one byte of entropy. However the virtio v0.9 spec does not mention such a requirement.
v0.9 was a draft. So nothing we have to care about.
The Arm Fixed Virtual Platform (FVP) implementation of virtio-rng always returns 8 bytes less of entropy than requested. If 8 bytes or less are requested, it will return 0 bytes. This behaviour makes U-Boot's virtio_rng_read() implementation go into an endless loop, hanging the system.
Work around this problem by always requesting 8 bytes more than needed, but only if a previous call to virtqueue_get_buf() returned 0 bytes.
This should never trigger on a v1.x spec compliant implementation, but fixes the hang on the Arm FVP.
Signed-off-by: Andre Przywara andre.przywara@arm.com Reported-by: Peter Hoyes peter.hoyes@arm.com
The bug should be fixed in the Arm Fixed Virtual Platform instead of working around it in U-Boot.
... and it is already fixed in the internal releases, but external users have to wait for the next public release in a few weeks, and then everyone would need to update as well. So I was hoping that we could just have this simple software fix? After all non-spec compliant hardware is quite common, and we have tons of quirks and workarounds for every kind of erratum or hardware problem. I don't think this one here is particularly intrusive. If you want, I can hide it behind a Kconfig option?
Our driver should return -EIO if the virtio host is not compliant.
That sounds a bit over the top to detect this and deliberately deny service, when we could just be robust and carry on anyways?
Cheers, Andre
Best regards
Heinrich
drivers/virtio/virtio_rng.c | 9 +++++++-- 1 file changed, 7 insertions(+), 2 deletions(-)
Unrelated to this patch, but do you know the hardware architecture of the ARM RNG? Is there one RNG unit per CPU or one for the whole SoC?
Architecturally and from a software perspective the ARMv8.5 FEAT_RNG feature is a system register, so per-core. Theoretically the availability could differ between cores, but the CPU ID feature registers are also per-core, so as long as we run on a single core, or always at least read from the same core, it's all good.
Now the architecture only describes the CPU instruction aspect of the feature, and establishes rules for the quality and spec conformance, but leaves the actual source of the entropy open to the integrator.
The manual in the Neoverse V1 core[1] (still not a SoC!) states that the actual entropy source is a memory mapped peripheral, its address being held in an internal, per-core register. So you can have one shared entropy source per SoC, or a private instance for each core, that's up to the actual integrator to design.
From the software perspective this shouldn't matter, though: the feature is "per-core", how this is backed is an implementation detail.
Cheers, Andre
[1] https://developer.arm.com/documentation/101427/0102/Functional-description/R...
diff --git a/drivers/virtio/virtio_rng.c b/drivers/virtio/virtio_rng.c index b85545c2ee5..786359a6e36 100644 --- a/drivers/virtio/virtio_rng.c +++ b/drivers/virtio/virtio_rng.c @@ -20,7 +20,7 @@ struct virtio_rng_priv { static int virtio_rng_read(struct udevice *dev, void *data, size_t len) { int ret;
unsigned int rsize;
unsigned int rsize = 1; unsigned char buf[BUFFER_SIZE] __aligned(4); unsigned char *ptr = data; struct virtio_sg sg;@@ -29,7 +29,12 @@ static int virtio_rng_read(struct udevice *dev, void *data, size_t len)
while (len) { sg.addr = buf;
sg.length = min(len, sizeof(buf));
/** Work around implementations which always return 8 bytes* less than requested, down to 0 bytes, which would* cause an endless loop otherwise.*/sg.length = min(rsize ? len : len + 8, sizeof(buf)); sgs[0] = &sg; ret = virtqueue_add(priv->rng_vq, sgs, 0, 1);-- 2.25.1
Regards, Simon