[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] Re: BUG: libxl vuart build order
Hi Stefano, On Thu, Oct 29, 2020 at 07:03:28PM -0700, Stefano Stabellini wrote: > + xen-devel and libxl maintainers > > In short, there is a regression in libxl with the ARM vuart introduced > by moving ARM guests to the PVH build. > > > On Thu, 29 Oct 2020, Takahiro Akashi wrote: > > On Wed, Oct 28, 2020 at 02:44:16PM -0700, Stefano Stabellini wrote: > > > On Wed, 28 Oct 2020, Takahiro Akashi wrote: > > > > On Tue, Oct 27, 2020 at 09:02:14AM +0900, Takahiro Akashi wrote: > > > > > On Mon, Oct 26, 2020 at 04:37:30PM -0700, Stefano Stabellini wrote: > > > > > > > > > > > > On Mon, 26 Oct 2020, Takahiro Akashi wrote: > > > > > > > Stefano, > > > > > > > > > > > > > > # I'm afraid that I have already bothered you with a lot of > > > > > > > questions. > > > > > > > > > > > > > > When I looked at Xen's vpl011 implementation, I found > > > > > > > CR (and LCHR) register is not supported. (trap may cause a data > > > > > > > abort). > > > > > > > On the other hand, for example, linux's pl011 driver surely > > > > > > > accesses CR (and LCHR) register. > > > > > > > So I guess that linux won't be able to use pl011 on a Xen guest vm > > > > > > > if vuart = "sbsa_uart". > > > > > > > > > > > > > > Is this a known issue or do I miss anything? > > > > > > > > > > > > Linux should definitely be able to use it, and in fact, I am using > > > > > > it > > > > > > with Linux in my test environment. > > > > > > > > > > > > I think the confusion comes from the name "vpl011": it is in fact > > > > > > not a > > > > > > full PL011 UART, but an SBSA UART. > > > > > > > > > > Yeah, I have noticed it. > > > > > > > > > > > SBSA UART only implements a subset of > > > > > > the PL011 registers. The compatible string is "arm,sbsa-uart", also > > > > > > see > > > > > > drivers/tty/serial/amba-pl011.c:sbsa_uart_probe. > > > > > > > > > > Looking closely into the details of implementation, I found > > > > > that all the accesses to unimplemented registers, including > > > > > CR, are deliberately avoided in sbsa part of linux driver. > > > > > > > > So I'm now trying to implement "sbsa-uart" driver on U-Boot > > > > by modifying the existing pl011 driver. > > > > (Please note the current xen'ized U-Boot utilises a para-virtualized > > > > console, i.e. with HVM_PARAM_CONSOLE_PFN.) > > > > > > > > So far my all attempts have failed. > > > > > > > > There are a couple of problems, and one of them is how we can > > > > access vpl011 port (from dom0). > > > > What I did is: > > > > - modify U-Boot's pl011 driver > > > > (I'm sure that the driver correctly handle a vpl011 device > > > > with regard of accessing a proper set of registers.) > > > > - start U-Boot guest with "vuart=sbsa_uart" by > > > > xl create uboot.cfg -c > > > > > > > > Then I have seen almost nothing on the screen. > > > > Digging into vpl011 implementation, I found that all the characters > > > > written DR register will be directed to a "backend domain" if a guest > > > > vm is launched by xl command. > > > > (In case of dom0less, the backend seems to be Xen itself.) > > > > > > > > As a silly experiment, I modified domain_vpl011_init() to always create > > > > a vpl011 device with "backend_in_domain == false". > > > > Then, I could see more boot messages from U-Boot, but still fails > > > > to use the device as a console, I mean, we will lose all the outputs > > > > after at some point and won't be able to type any keys (at a command > > > > prompt). > > > > (This will be another problem on U-Boot side.) > > > > > > > > My first question here is how we can configure and connect a console > > > > in this case? > > > > Should "xl create -c" or "xl console -t vuart" simply work? > > > > > > "xl create -c" creates a guest and connect to the primary console which > > > is the PV console (i.e. HVM_PARAM_CONSOLE_PFN.) > > > > So in case of vuart, it (console) doesn't work? > > (Apparently, "xl create" doesn't take '-t' option.) > > > > > To connect to the emulated sbsa uart you need to pass -t vuart. So yes, > > > "xl console -t vuart domain_name" should get you access to the emulated > > > sbsa uart. The sbsa uart can also be exposed to dom0less guests; you get > > > their output by using CTRL-AAA to switch to right domU console. > > > > > > You can add printks to xen/arch/arm/vpl011.c in Xen to see what's > > > happening on the Xen side. vpl011.c is the emulator. > > > > I'm sure that write to "REG_DR" register is caught by Xen. > > What I don't understand is > > if back_in_domain -> no outputs > > if !back_in_domain -> can see outputs > > > > (As you know, if a guest is created by xl command, back_in_domain > > is forcedly set to true.) > > > > I looked into xenstore and found that "vuart/0/tty" does not exist, > > but "console/tty" does. > > How can this happen for vuart? > > (I clearly specified, vuart = "sbsa_uart" in Xen config.) > > It looks like we have a bug :-( > > I managed to reproduce the issue. The problem is a race in libxl. > > tools/libxc/xc_dom_arm.c:alloc_magic_pages is called first, setting > dom->vuart_gfn. Then, libxl__build_hvm should be setting > state->vuart_gfn to dom->vuart_gfn (like libxl__build_pv does) but it > doesn't. Thank you for the patch. I confirmed that sbsa-uart driver on U-Boot now works. === after "xl console -t vuart" === U-Boot 2020.10-00777-g10cf956a26ba (Oct 29 2020 - 19:31:29 +0900) xenguest Xen virtual CPU Model: XENVM-4.15 DRAM: 128 MiB In: sbsa-pl011 Out: sbsa-pl011 Err: sbsa-pl011 xenguest# dm tree Class Index Probed Driver Name ----------------------------------------------------------- root 0 [ + ] root_driver root_driver firmware 0 [ ] psci |-- psci serial 0 [ + ] serial_pl01x |-- sbsa-pl011 pvblock 0 [ ] pvblock `-- pvblock === If possible, I hope that "xl create -c" command would accept "-t vuart" option (or it should automatically selects uart type from the config). -Takahiro Akashi > > --- > > libxl: set vuart_gfn in libxl__build_hvm > > Setting vuart_gfn was missed when switching ARM guests to the PVH build. > Like libxl__build_pv, libxl__build_hvm should set state->vuart_gfn to > dom->vuart_gfn. > > Signed-off-by: Stefano Stabellini <stefano.stabellini@xxxxxxxxxx> > > diff --git a/tools/libxl/libxl_dom.c b/tools/libxl/libxl_dom.c > index f8661e90d4..36fe8915e7 100644 > --- a/tools/libxl/libxl_dom.c > +++ b/tools/libxl/libxl_dom.c > @@ -1184,6 +1184,7 @@ int libxl__build_hvm(libxl__gc *gc, uint32_t domid, > LOG(ERROR, "hvm build set params failed"); > goto out; > } > + state->vuart_gfn = dom->vuart_gfn; > > rc = hvm_build_set_xs_values(gc, domid, dom, info); > if (rc != 0) {
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