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Re: [Xen-devel] [PATCH 5/6] x86/AMD: Fix handling of FPU pointer on Zen hardware
- To: Pu Wen <puwen@xxxxxxxx>, Xen-devel <xen-devel@xxxxxxxxxxxxx>
- From: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
- Date: Sat, 29 Dec 2018 13:02:27 +0000
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- Cc: Wei Liu <wei.liu2@xxxxxxxxxx>, Jan Beulich <JBeulich@xxxxxxxx>, Roger Pau Monné <roger.pau@xxxxxxxxxx>
- Delivery-date: Sat, 29 Dec 2018 13:02:57 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
- Openpgp: preference=signencrypt
On 29/12/2018 09:52, Pu Wen wrote:
> On 2018/12/28 20:40, Andrew Cooper wrote:
>> AMD hardware before Zen doesn't safe/restore the FPU error pointers
>> unless an unmasked FPU exception is pending. Zen processors have a
>> feature bit indicating that this (mis)behaviour no longer exists.
>>
>> Express the common logic in terms of cpu_bug_fpu_err_ptr as Hygon
>> processors (being Zen derivatives) won't inherit this behaviour.
> ...
>> diff --git a/xen/arch/x86/xstate.c b/xen/arch/x86/xstate.c
>> index 15edd5d..7ca5684 100644
>> --- a/xen/arch/x86/xstate.c
>> +++ b/xen/arch/x86/xstate.c
>> @@ -369,15 +369,13 @@ void xrstor(struct vcpu *v, uint64_t mask)
>> unsigned int faults, prev_faults;
>>
>> /*
>> - * AMD CPUs don't save/restore FDP/FIP/FOP unless an exception
>> + * Some CPUs don't save/restore FDP/FIP/FOP unless an exception
>> * is pending. Clear the x87 state here by setting it to fixed
>> * values. The hypervisor data segment can be sometimes 0 and
>> * sometimes new user value. Both should be ok. Use the FPU saved
>> * data block as a safe address because it should be in L1.
>> */
>> - if ( (mask & ptr->xsave_hdr.xstate_bv & X86_XCR0_FP) &&
>> - !(ptr->fpu_sse.fsw & ~ptr->fpu_sse.fcw & 0x003f) &&
>> - boot_cpu_data.x86_vendor == X86_VENDOR_AMD )
>> + if ( cpu_bug_fpu_err_ptr )
>> asm volatile ( "fnclex\n\t" /* clear exceptions */
>> "ffree %%st(7)\n\t" /* clear stack tag */
>> "fildl %0" /* load to clear state */
> It seems that the functions fpu_fxrstor and fpu_fxsave have the same
> problem, even though they are not called on Hygon Dhyana platform.
> Should them be fixed in the same way?
Oops yes - they should be fixed. They will get used if the user passes
"xsave=0" on the command line.
~Andrew
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