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Re: [Qemu-devel] [PATCH v2 3/6] pvrdma: check number of pages when creat

From: P J P
Subject: Re: [Qemu-devel] [PATCH v2 3/6] pvrdma: check number of pages when creating rings
Date: Tue, 18 Dec 2018 00:17:59 +0530 (IST)

  Hello Yuval,

+-- On Sun, 16 Dec 2018, Yuval Shaia wrote --+
| With this patch the last step fails, the guest OS hangs, trying to probably 
| unload pvrdma driver and finally gave up after 3 minutes.

| Anyways with debug turned on i have noticed that there is one case that 
| devices gets 129 nchunks (i think in MPI) while your patch limits it to 128.
| >From pvrdma source code  we can see that first page is dedicated to ring
| state, this means that it maybe correct that 128 is the limit but we
| should check that nchunks does not exceed 129, not 128.
| What do you think?


the vmw_pvrdma kernel driver also seems to set MAX_FAST_REG_PAGE = 128.

| Ie. to replace this line from create_cq_ring
| +    if (!nchunks || nchunks > PVRDMA_MAX_FAST_REG_PAGES) {
| with this
| +    if (!nchunks || nchunks > PVRDMA_MAX_FAST_REG_PAGES + 1) {
| Let me know your opinion.

While it may help to fix the regression. I'm not sure it's a right fix.
129 seems a little odd number to have as limit.

Is it possible MPI is erring in getting 129 chunks?

IMO it's better to confirm the right value for 'MAX_FAST_REG_PAGES', before 
going with > PVRDMA_MAX_FAS_REG_PAGES(=128) + 1.

Thank you.
Prasad J Pandit / Red Hat Product Security Team
47AF CE69 3A90 54AA 9045 1053 DD13 3D32 FE5B 041F

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