[PATCH] efi: fix semihosting EFI payload booting
Andre Przywara
andre.przywara at arm.com
Thu May 11 02:00:16 CEST 2023
On Wed, 10 May 2023 23:19:33 +0200
Heinrich Schuchardt <xypron.glpk at gmx.de> wrote:
Hi,
> On 5/10/23 19:26, Andre Przywara wrote:
> > On Wed, 10 May 2023 17:58:06 +0200
> > Heinrich Schuchardt <xypron.glpk at gmx.de> wrote:
> >
> > Hi,
> >
> >> On 5/10/23 16:13, Andre Przywara wrote:
> >>> At the moment any naive attempt to boot an EFI payload that has just
> >>> been loaded via "hostfs" (sandbox or semihosting) is met by a rather
> >>> confusing error message:
> >>> ===========
> >>> VExpress64# load hostfs - $kernel_addr_r Image
> >>> 52752896 bytes read in 8 ms (6.1 GiB/s)
> >>> VExpress64# bootefi $kernel_addr_r
> >>> No UEFI binary known at 0x80080000
> >>> ===========
> >>> Actually explicitly providing the filesize:
> >>> VExpress64# bootefi $kernel_addr_r:$filesize
> >>> works around that problem, but the issue lies deeper: the call to
> >>> efi_set_bootdev() (as done by the generic load code) bails out at some
> >>> point, leaving the image_addr and image_size variables unset, which
> >>> triggers this message. The problem seems to be that "-" is not
> >>> understood by the code creating an UEFI device path. We could try to fix
> >>> just that, but actually semihosting seems to have some explicit support
> >>> in UEFI (at least it does in EDK II): there is a separate GUID for it,
> >>> and hostfs is significantly different in some aspects to justify special
> >>> handling.
> >>>
> >>> Check for the device name being "hostfs" and create a specific UEFI device
> >>> path for semihosting in this case. This uses the GUID used by EDK II for
> >>> almost 15 years.
> >>> This fixes the above load/bootefi sequence without requiring an explicit
> >>> file size argument.
> >>>
> >>> Signed-off-by: Andre Przywara <andre.przywara at arm.com>
> >>
> >> Could you, please, indicate how to invoke QEMU with semihosting enabled.
> >> This information needs to be added to doc/usage/semihosting.rst.
> >
> > It's already there:
> > https://u-boot.readthedocs.io/en/latest/usage/semihosting.html#qemu
>
> This does not tell me which arguments I should pass to qemu-system-aarch64.
I would say that's out of scope for this document, and is explained in
doc/boards/emulation/qemu-arm.rst. A minimum working example is:
$ qemu-system-aarch64 -M virt -cpu cortex-a57 -nographic -bios u-boot.bin
> > It's mostly just "-semihosting" on the QEMU side, but it's not
> > enabled by default in the QEMU defconfig, because it *was* needed
> > to be provided in sync with the QEMU option. It's a debug feature,
> > so there is no good discovery mechanism, really. I think Sean made
> > this work, with this autodetect via trap feature, so we might want
> > to enable this option now, at least for the filesystem part.
>
> If it is missing in Ubuntu's QEMU, please, indicate how I can build a
> QEMU that allows testing. This information needs to go into
> semihosting.rst.
It's all there and it works on my 20.04 QEMU build out of the box. What
I meant is that -semihosting is a QEMU command line *option*, and a very
optional one, so to speak, as it's more an ARM low level debugger
technology than anything else, and it was just adopted by QEMU. And
until recently you had to make sure that you give that option to QEMU
if you enable semihosting support in U-Boot, otherwise it would crash.
For more details see Sean's FOSDEM talk:
https://fosdem.org/2023/schedule/event/semihosting_uboot/
> > Regardless I was doing those experiments with the FVP fastmodel, which is
>
> It seems FVP's are not open source
> (https://developer.arm.com/Tools%20and%20Software/Fixed%20Virtual%20Platforms).
>
> We need some open source solution for testing the suggested changes.
Sure, I just mentioned this because the models are just the reference
implementation and natural habitat for semihosting: because it's
enabled by default there, and it's an ancient ARM technology.
If you want to test it, just use your favourite QEMU command line (or
the one I mention above) and add "-semihosting". Then load your EFI app:
=> load hostfs - $kernel_addr_r app.efi
=> bootefi $kernel_addr_r
Cheers,
Andre
>
> Best regards
>
> Heinrich
>
> > the most original semihosting provider, I think, and always enables
> > semihosting.
> >
> >> Can there be multiple semihosting block devices?
> >
> > There is no semihosting *block device*, it's a pure filesystem interface.
> > And there is conceptually only one instance of that: it's basically a
> > hook in the debugger (or emulation software) to provide some kind of
> > syscall interface, triggered by a certain instruction ("brk" on AArch64).
> > This allows some very limited access to the host filesystem - read files,
> > write files, but no listing, for instance.
> > It is super convenient to launch the model and load the kernel (or
> > whatever) directly from your build machine's filesystem - especially if
> > you do bisects or trial-and-error experiments.
> >
> >>> ---
> >>> lib/efi_loader/efi_device_path.c | 34 ++++++++++++++++++++++++++++++++
> >>> 1 file changed, 34 insertions(+)
> >>>
> >>> diff --git a/lib/efi_loader/efi_device_path.c b/lib/efi_loader/efi_device_path.c
> >>> index e2e98a39be1..b6d2074dd70 100644
> >>> --- a/lib/efi_loader/efi_device_path.c
> >>> +++ b/lib/efi_loader/efi_device_path.c
> >>> @@ -1079,6 +1079,35 @@ struct efi_device_path *efi_dp_from_uart(void)
> >>> return buf;
> >>> }
> >>>
> >>> +#define SEMIHOSTING_GUID \
> >>> + EFI_GUID(0xc5b9c74a, 0x6d72, 0x4719, \
> >>> + 0x99, 0xab, 0xc5, 0x9f, 0x19, 0x90, 0x91, 0xeb)
> >>
> >> Can semihosting be moved to the driver model?
> >
> > Mmh, I am not sure this is a thing, since there is no block device, it's a
> > filesystem. Which is part of the problem, I believe, and probably also a
> > good reason to treat it separately.
> >
> >> Then we could create a generic catch all for device path nodes for all
> >> uclasses.
> >
> > Need to digest that idea ....
> >
> > Cheers,
> > Andre
> >
> >>
> >> Best regards
> >>
> >> Heinrich
> >>
> >>> +
> >>> +struct efi_device_path *efi_dp_from_semihosting(void)
> >>> +{
> >>> + const struct efi_device_path_vendor smh_vendor = {
> >>> + .dp = {
> >>> + .type = DEVICE_PATH_TYPE_HARDWARE_DEVICE,
> >>> + .sub_type = DEVICE_PATH_SUB_TYPE_VENDOR,
> >>> + .length = sizeof(smh_vendor),
> >>> + },
> >>> + .guid = SEMIHOSTING_GUID,
> >>> + };
> >>> + void *buf, *pos;
> >>> + size_t dpsize = sizeof(smh_vendor) + sizeof(END);
> >>> +
> >>> + buf = efi_alloc(dpsize);
> >>> + if (!buf)
> >>> + return NULL;
> >>> + pos = buf;
> >>> + memcpy(pos, &smh_vendor, sizeof(smh_vendor));
> >>> + pos += sizeof(smh_vendor);
> >>> +
> >>> + memcpy(pos, &END, sizeof(END));
> >>> +
> >>> + return buf;
> >>> +}
> >>> +
> >>> #ifdef CONFIG_NETDEVICES
> >>> struct efi_device_path *efi_dp_from_eth(void)
> >>> {
> >>> @@ -1210,6 +1239,11 @@ efi_status_t efi_dp_from_name(const char *dev, const char *devnr,
> >>> *device = efi_dp_from_mem(EFI_RESERVED_MEMORY_TYPE,
> >>> (uintptr_t)image_addr,
> >>> image_size);
> >>> + } else if (!strcmp(dev, "hostfs")) {
> >>> + efi_get_image_parameters(&image_addr, &image_size);
> >>> +
> >>> + if (device)
> >>> + *device = efi_dp_from_semihosting();
> >>> } else {
> >>> part = blk_get_device_part_str(dev, devnr, &desc, &fs_partition,
> >>> 1);
> >>
> >
>
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