On 11/12/23 12:53, Andre Przywara wrote:
On Sat, 11 Nov 2023 20:08:36 -0700 Simon Glass sjg@chromium.org wrote:
Hi,
On Fri, 10 Nov 2023 at 07:16, Andre Przywara andre.przywara@arm.com 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.
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
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.
Would it make sense to model this as a memory-mapped peripheral under /soc (perhaps one without an address?)
No, absolutely not. What I mentioned above is a somewhat hidden implementation detail of that *particular core*. One big reason for having those architected *system registers* is to do away with all those implementation specific ways to access an entropy source, and make this dead easy for software (including userland!) to use it: Check the CPU ID register, read the sysreg. No prior knowledge required.
I now deeply regret sending this Armv8.5 RNG driver. I have an alternative solution, just got distracted later this week to finish this up. Let's have a discussion there, or we find a way to probe UCLASS_RNG drivers other than through devicetree nodes. If U-Boot really insists on matching drivers to DT nodes 1:1, that's a really limiting design decision, and we should not proliferate this by shoehorning everyone and their dog into devicetree.
For the RISC-V Zkr driver Tom said we cannot expect QEMU and Linux to change how the device-tree is set up to model CPU registers and the usage of U_BOOT_DRVINFO() for these architecturally defined registers is fine.
I would assume the same in the ARM case.
Best regards
Heinrich
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