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Re: [Xen-devel] [PATCH 2/2] xen/pv: Call get_cpu_address_sizes to set x86_virt/phys_bits

On 07/23/2018 11:04 AM, Boris Ostrovsky wrote:
On 07/22/2018 11:57 AM, M. Vefa Bicakci wrote:
On 07/21/2018 07:17 PM, M. Vefa Bicakci wrote:
On 07/21/2018 05:25 PM, Boris Ostrovsky wrote:
On 07/21/2018 03:49 PM, M. Vefa Bicakci wrote:
diff --git a/arch/x86/xen/enlighten_pv.c b/arch/x86/xen/enlighten_pv.c
index 439a94bf89ad..87afb000142a 100644
--- a/arch/x86/xen/enlighten_pv.c
+++ b/arch/x86/xen/enlighten_pv.c
@@ -1257,6 +1257,7 @@ asmlinkage __visible void __init
       /* Work out if we support NX */
+    get_cpu_address_sizes(&boot_cpu_data);

Have you observed any problems without this call? get_cpu_cap() is only
called here to set X86_FEATURE_NX, and is then called again, together
with get_cpu_address_sizes(), from early_identify_cpu().

Thank you for the reviews! Without the call to get_cpu_address_sizes,
paravirtualized virtual machines do not boot up kernels with versions
4.17 and up at all; this includes dom0 and domU. No domU logs are
generated in dom0's /var/log/xen/console/ directory either, despite
having earlyprintk=xen on the kernel command line for my test domU.

Hello Boris,

I debugged this further with a debugging version of Xen (so that I can
get early kernel print-outs via the "xen_raw_console_write" function),
and I found the root cause of the boot up failure.

In summary, the issue is due to the following call path in version
4.17 (and higher, I assume), which the kernel goes through /only/ when

         physaddr.h::phys_addr_valid // uses boot_cpu_data.x86_phys_bits

The return value of phys_addr_valid is used with the VIRTUAL_BUG_ON
which evaluates to BUG_ON in case CONFIG_DEBUG_VIRTUAL is enabled.

Ah, that's why it hasn't been detected.

It looks like the call to get_cpu_address_size is required in the
xen_start_kernel function. Perhaps there is a more elegant way to
resolve this issue as well.

Another approach could be to check in the phys_addr_valid function
boot_cpu_data.x86_phys_bits has been initialized or not, I think, but
I am
not sure about the correctness of this approach.

No, I think your patch is good. The only thing I'd suggest is to move
the call a few lines down. The way it is placed now may create
impression that we are calling get_cpu_address_sizes() to figure out NX

Sorry for the delay, and thank you for your comments! I will
send an updated version of this patch in a few moments.


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