On 10/09/2019 10:29, Roger Pau Monné wrote:
> On Mon, Sep 09, 2019 at 04:51:24PM +0100, Andrew Cooper wrote:
>> 7a0 is an integer field, not a mask - taking the logical and of the hardware
>> and policy values results in nonsense. Instead, take the policy value
>> directly.
>>
>> Signed-off-by: Andrew Cooper <andrew.coop...@citrix.com>
>> ---
>> CC: Jan Beulich <jbeul...@suse.com>
>> CC: Wei Liu <w...@xen.org>
>> CC: Roger Pau Monné <roger....@citrix.com>
>>
>> Even Rome hardware has 7[0].eax still as 0, and there is no sensible reason
>> to
>> set max_subleaf higher at this point, so this is only a latent bug for now.
>> ---
>> xen/arch/x86/domctl.c | 13 +++++++++----
>> 1 file changed, 9 insertions(+), 4 deletions(-)
>>
>> diff --git a/xen/arch/x86/domctl.c b/xen/arch/x86/domctl.c
>> index 1e98fc8009..35ad8cb51c 100644
>> --- a/xen/arch/x86/domctl.c
>> +++ b/xen/arch/x86/domctl.c
>> @@ -218,11 +218,16 @@ static int update_domain_cpuid_info(struct domain *d,
>> if ( is_pv_domain(d) && ((levelling_caps & LCAP_7ab0) == LCAP_7ab0)
>> )
>> {
>> uint64_t mask = cpuidmask_defaults._7ab0;
>> - uint32_t eax = ctl->eax;
>> - uint32_t ebx = p->feat._7b0;
>>
>> - if ( boot_cpu_data.x86_vendor & (X86_VENDOR_AMD |
>> X86_VENDOR_HYGON) )
>> - mask &= ((uint64_t)eax << 32) | ebx;
>> + /*
>> + * Leaf 7[0].eax is max_subleaf, not a feature mask. Take it
>> + * wholesale from the policy, but clamp the features in 7[0].ebx
>> + * per usual.
>> + */
>> + if ( boot_cpu_data.x86_vendor &
>> + (X86_VENDOR_AMD | X86_VENDOR_HYGON) )
>> + mask = (((uint64_t)p->feat.max_subleaf << 32) |
>> + ((uint32_t)mask | p->feat._7b0));
> Why do you set the high bits of the mask (63:30) with the max subleaf?
63:32
> According to the document I have bits 63:30 are reserved, and that
> seems to match the expected CPUID return value, that lists CPUID
> Fn0000_0007_EAX_x0 content as reserved.
Yes, but reserved doesn't mean "will #GP". As I said on IRC, this MSR
*is* the value which gets forwarded into a CPUID invocation, and setting
max_subleaf to 0xdead does work fine.
The point here is that in the future, on hardware capable of
max_subleaf=2 and being levelled to max_subleaf=1, the value observed in
CPUID should be 1, not 0.
The latter is what the current logic does, and is broken. The CPUID
derivation logic will ensure that policy max_subleaf <= hardware
max_subleaf.
~Andrew
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