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hi  ,

>I assume it's t= he BIOS's driver and it's choosing a range in low memory,
>but still I'm not = sure why its DMA is racing against the PAM update
>(which is done ver= y early).  Felix, do you know the answer?


This bug is trigger= ed by make_bios_readonly() in seabios. Make_bios_readonly() will write pam = register.

And ehci_setup= () is executed before make_bios_readonly(). After initializing EHCI in= seabios, it will

continuously send d= ma cycles. So when write pam register, this bug appeared.


= In addition, before write pam registers= , flatview_simplify() has merged a very large range.For example,

= this large range is 0xc0000-0xbfffffff.=  So even if EHCI is configured to not allocate buffers in low memory,<= /span>

= this bug will still occur.Thanks.

=

= Best regards

= Felixcui-oc

=B7=A2=BC=FE=C8=CB: Paolo= Bonzini <pbonzini@redhat.com>
=B7=A2=CB=CD=CA=B1=BC=E4: 2020=C4=EA10=D4=C222=C8=D5 3:50:15
=CA=D5=BC=FE=C8=CB: Alex Williamson
=B3=AD=CB=CD: FelixCui-oc; Richard Henderson; Eduardo Habkost; qemu-= devel@nongnu.org; RockCui-oc; Tony W Wang-oc; CobeChen-oc
=D6=F7=CC=E2: Re: [PATCH 1/1] Skip flatview_simplify() for cpu vendo= r zhaoxin
 
On 21/10/20 20:49, Alex Williamson wrote:
> I can understand the general benefit of flatview_simplify(), but I
> wonder if the best short term solution is to skip operating on the x86=
> PAM range, which I understand to be a small number of memory chunks > below 1MB.

I'd rather remove flatview_simplify altogether, it probably triggers
relatively rarely.  Possibly do not let it operate on RAM/ROM regions,=
only on I/O regions.

> I might also wonder why the EHCI controller on this
> platform is choosing that range for DMA.

I assume it's the BIOS's driver and it's choosing a range in low memory, but still I'm not sure why its DMA is racing against the PAM update
(which is done very early).  Felix, do you know the answer?

Paolo

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