Hi Ayan, On 22/11/2023 20:00, Ayan Kumar Halder wrote: > Hi Amit/All, > > We came across this scenario and would be helpful if you can provide > some pointers > > > Linux running as Dom0 on Xen hypervisor with HVC_DCC = y and HVC_XEN = y > on Arm64 platform. > > This causes a crash when Dom0 tries to access "DBGDTRTX_EL0" register, > it traps to Xen. > > Xen does not emulate this register so it crashes. > > |Logs - https://paste.debian.net/1298983/| > > | > | > > |My aim is to avoid the crash and let Xen boot Dom0 even though there > might not be a debug console available.| > > |So, I am trying to add emulation for |"DBGDTRTX_EL0" register in Xen. > > > As a quick trial (may be not the perfect solution), I am trying to > emulate this register as "read as zero/write ignore" (similar to KVM). > > However, I could not see logs to the Xen console (ie HVC0). > > > So my question is > > 1. Is it possible in Linux to probe more than one HVC console so that > Linux can put the same logs in HVC_DCC and HVC_XEN drivers ? > > So that the user can always see the logs in the default Xen console (ie > hvc0) even if the debug console is not present. If you have both HVC DCC and Xen enabled and DCC gets assigned hvc0, just use console=hvc1 to see the messages from Xen. > > > Another possible alternative I am exploring is to enable trapping for > read of MDCCSR_EL0 in Xen, so that Xen returns with MDCCSR_EL0.TXfull > set to 1. > > static int hvc_dcc_put_chars(uint32_t vt, const char *buf, int count) > { > int i; > > for (i = 0; i < count; i++) { > while (__dcc_getstatus() & DCC_STATUS_TX) > cpu_relax(); <<<<<<<<<<----------- So this will be invoked. FWICS, this won't be invoked given that in init call hvc_dcc_check() will return false due to condition "if (!(__dcc_getstatus() & DCC_STATUS_TX))" being false. This is the whole point to make Linux return -ENODEV quickly.
~Michal
