答复: [PATCH 1/1] Skip flatview_simplify() for cpu vendor zhaoxin

[FelixCui-oc]

hi  ,

>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?

This bug is triggered 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 dma 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,

this bug will still occur.Thanks.

Best regards

Felixcui-oc

---

发件人: Paolo Bonzini <pbonzini@redhat.com>
发送时间: 2020年10月22日 3:50:15
收件人: Alex Williamson
抄送: FelixCui-oc; Richard Henderson; Eduardo Habkost; qemu-devel@nongnu.org; RockCui-oc; Tony W Wang-oc; CobeChen-oc
主题: Re: [PATCH 1/1] Skip flatview_simplify() for cpu vendor 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