Re: [Xen-devel] [PATCH v4 2/7] x86/hyperv: setup hypercall page

[Jan Beulich <jbeulich@xxxxxxxx>]

On 28.01.2020 16:30, Wei Liu wrote:
> On Thu, Jan 23, 2020 at 12:18:41PM +0100, Jan Beulich wrote:
>> On 22.01.2020 21:23, Wei Liu wrote:
>>> --- a/xen/arch/x86/e820.c
>>> +++ b/xen/arch/x86/e820.c
>>> @@ -36,6 +36,22 @@ boolean_param("e820-verbose", e820_verbose);
>>>  struct e820map e820;
>>>  struct e820map __initdata e820_raw;
>>>  
>>> +static unsigned int find_phys_addr_bits(void)
>>> +{
>>> +    uint32_t eax;
>>> +    unsigned int phys_bits = 36;
>>> +
>>> +    eax = cpuid_eax(0x80000000);
>>> +    if ( (eax >> 16) == 0x8000 && eax >= 0x80000008 )
>>> +    {
>>> +        phys_bits = (uint8_t)cpuid_eax(0x80000008);
>>> +        if ( phys_bits > PADDR_BITS )
>>> +            phys_bits = PADDR_BITS;
>>> +    }
>>> +
>>> +    return phys_bits;
>>> +}
>>
>> Instead of this, how about pulling further ahead the call to
>> early_cpu_init() in setup.c? (Otherwise the function wants to
>> be __init at least.)
> 
> I can certainly try that, but that would require modifying e820.c
> nonetheless because we can drop the cpuid invocation here if the move is
> successful.

Right, but this could then be a separate, follow-on cleanup
patch aiui.

>>> --- a/xen/arch/x86/guest/hyperv/hyperv.c
>>> +++ b/xen/arch/x86/guest/hyperv/hyperv.c
>>> @@ -18,17 +18,27 @@
> [...]
>>> @@ -72,6 +82,43 @@ const struct hypervisor_ops *__init hyperv_probe(void)
>>>      return &ops;
>>>  }
>>>  
>>> +static void __init setup_hypercall_page(void)
>>> +{
>>> +    union hv_x64_msr_hypercall_contents hypercall_msr;
>>> +    union hv_guest_os_id guest_id;
>>> +    unsigned long mfn;
>>> +
>>> +    rdmsrl(HV_X64_MSR_GUEST_OS_ID, guest_id.raw);
>>> +    if ( !guest_id.raw )
>>> +    {
>>> +        guest_id.raw = generate_guest_id();
>>> +        wrmsrl(HV_X64_MSR_GUEST_OS_ID, guest_id.raw);
>>> +    }
>>> +
>>> +    rdmsrl(HV_X64_MSR_HYPERCALL, hypercall_msr.as_uint64);
>>> +    if ( !hypercall_msr.enable )
>>> +    {
>>> +        mfn = ((1ull << paddr_bits) - 1) >> HV_HYP_PAGE_SHIFT;
>>
>> Along the lines of the abstracting-away request above: How is
>> anyone to notice what else needs changing if it is decided
>> that this page gets moved elsewhere?
> 
> I don't have a good answer to this other than documenting. It is
> probably as fragile as livepatch or pcpu stub.

At least macro-ize it then, so that all use sites can be easily
identified.

Jan

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