linux-kernel - Re: [PATCH] iommu/amd: Do not re-fetch iommu->cmd_buf

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Date:   Wed, 30 Oct 2019 10:20:14 +0100
From:   Joerg Roedel <jroedel@...e.de>
To:     Denys Vlasenko <dvlasenk@...hat.com>
Cc:     Tom Lendacky <thomas.lendacky@....com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] iommu/amd: Do not re-fetch iommu->cmd_buf_tail

On Thu, Oct 24, 2019 at 02:54:10PM +0200, Denys Vlasenko wrote:
> The compiler is not smart enough to realize that iommu->cmd_buf_tail
> can't be modified across memcpy:
> 
> 41 8b 45 74          mov    0x74(%r13),%eax   # iommu->cmd_buf_tail
> 44 8d 78 10          lea    0x10(%rax),%r15d  # += sizeof(*cmd)
> 41 81 e7 ff 1f 00 00 and    $0x1fff,%r15d     # %= CMD_BUFFER_SIZE
> 49 03 45 68          add    0x68(%r13),%rax   # target = iommu->cmd_buf + iommu->cmd_buf_tail
> 45 89 7d 74          mov    %r15d,0x74(%r13)  # store to iommu->cmd_buf_tail
> 49 8b 34 24          mov    (%r12),%rsi       # memcpy
> 49 8b 7c 24 08       mov    0x8(%r12),%rdi    # memcpy
> 48 89 30             mov    %rsi,(%rax)       # memcpy
> 48 89 78 08          mov    %rdi,0x8(%rax)    # memcpy
> 49 8b 55 38          mov    0x38(%r13),%rdx   # iommu->mmio_base
> 41 8b 45 74          mov    0x74(%r13),%eax   # redundant load of iommu->cmd_buf_tail
> ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
> 89 82 08 20 00 00    mov    %eax,0x2008(%rdx) # writel
> 
> CC: Tom Lendacky <thomas.lendacky@....com>
> CC: Joerg Roedel <jroedel@...e.de>
> CC: linux-kernel@...r.kernel.org
> Signed-off-by: Denys Vlasenko <dvlasenk@...hat.com>
> ---
>  drivers/iommu/amd_iommu.c | 13 +++++++------
>  1 file changed, 7 insertions(+), 6 deletions(-)

Applied, thanks.