iommu的兩大功能，一個是dma 映射，一個是中斷的映射。本文簡單描述對應的內核參數解析。
1、在iommu 內核參數的解析的 x86架構整體部分
iommu= [x86] off
force
noforce
biomerge
panic
nopanic
merge
nomerge
soft //caq:沒有iommu硬件，使用軟件來達到iommu的功能，主要是針對bounce buffering
pt [x86]
nopt [x86]
nobypass [PPC/POWERNV] Disable IOMMU bypass, using IOMMU for PCI devices.
對應的代碼解析為：
static __init int iommu_setup(char *p)
{
iommu_merge = 1;//caq:默認使能sg合並
if (!p)
return -EINVAL;
while (*p) {
if (!strncmp(p, "off", 3))//caq:不要iommu功能，不管是硬件的iommu還是 swiotlb
no_iommu = 1;
/* gart_parse_options has more force support */
if (!strncmp(p, "force", 5))//caq: 強制使用硬件IOMMU,即使硬件可能有缺陷或者機器內存很少
force_iommu = 1;
if (!strncmp(p, "noforce", 7)) {//caq:默認就是非強制 ，此時 會設置 force_iommu 為0
iommu_merge = 0;
force_iommu = 0;
}
if (!strncmp(p, "biomerge", 8)) {//caq:和 merge 參數一樣
iommu_merge = 1;
force_iommu = 1;
}
if (!strncmp(p, "panic", 5))//caq:dma_map_area overflow時 是否觸發panic
panic_on_overflow = 1;
if (!strncmp(p, "nopanic", 7))//caq:dma_map_area overflow 時是否關閉panic
panic_on_overflow = 0;
if (!strncmp(p, "merge", 5)) {//caq:會強制iommu，並且要求"scatter-gather"合並
iommu_merge = 1;
force_iommu = 1;
}
if (!strncmp(p, "nomerge", 7))//caq:禁止"scatter-gather"合並,主要用於分散聚集dma
iommu_merge = 0;
if (!strncmp(p, "forcesac", 8))//caq:過期的參數了
pr_warn("forcesac option ignored.\n");
if (!strncmp(p, "allowdac", 8))//caq:同上
pr_warn("allowdac option ignored.\n");
if (!strncmp(p, "nodac", 5))//caq:同上
pr_warn("nodac option ignored.\n");
if (!strncmp(p, "usedac", 6)) {//caq: usedac 一般也不配置，這樣disable_dac_quirk 默認為0
disable_dac_quirk = true;//caq:將32位PCI地址用兩個時鍾周期推入64位地址，這就是DAC的作用。
return 1;
}
#ifdef CONFIG_SWIOTLB
if (!strncmp(p, "soft", 4))//caq:使用 swiotlb ，主要用於bounce buffering，拷貝+sync
swiotlb = 1;
#endif
if (!strncmp(p, "pt", 2))
iommu_pass_through = 1;//caq:設置為pt，則使能 iommu_pass_through
if (!strncmp(p, "nopt", 4))//caq:設置了nopt，則關閉 iommu_pass_through
iommu_pass_through = 0;
gart_parse_options(p);
#ifdef CONFIG_CALGARY_IOMMU
if (!strncmp(p, "calgary", 7))//caq:IBM Calgary 的支持，我們一般用不到
use_calgary = 1;
#endif /* CONFIG_CALGARY_IOMMU */
p += strcspn(p, ",");
if (*p == ',')
++p;
}
return 0;
}
另外還有一個參數
iommu.passthrough= [ARM64] Configure DMA to bypass the IOMMU by default.
Format: { "0" | "1" } 0 - Use IOMMU translation for DMA. 1 - Bypass the IOMMU for DMA.
unset - Use IOMMU translation for DMA.
雖然寫的是arm64，但是實際代碼的作用區域是：
static int __init iommu_set_def_domain_type(char *str)
{
bool pt;
int ret;
ret = kstrtobool(str, &pt);
if (ret)
return ret;
iommu_def_domain_type = pt ? IOMMU_DOMAIN_IDENTITY : IOMMU_DOMAIN_DMA;//為0則uses IOMMU
return 0;
}
early_param("iommu.passthrough", iommu_set_def_domain_type);//caq:是否設定iommu.default_domain的是否bypass---注意，並不一定指arm64
2、在iommu 內核參數的解析的 intel 硬件部分：
intel_iommu= [DMAR] Intel IOMMU driver (DMAR) option
on
Enable intel iommu driver.
off
Disable intel iommu driver.
igfx_off [Default Off]
By default, gfx is mapped as normal device. If a gfx
device has a dedicated DMAR unit, the DMAR unit is
bypassed by not enabling DMAR with this option. In
this case, gfx device will use physical address for
DMA.
forcedac [x86_64]
With this option iommu will not optimize to look
for io virtual address below 32-bit forcing dual
address cycle on pci bus for cards supporting greater
than 32-bit addressing. The default is to look
for translation below 32-bit and if not available
then look in the higher range.
strict [Default Off]
With this option on every unmap_single operation will
result in a hardware IOTLB flush operation as opposed
to batching them for performance.
sp_off [Default Off]
By default, super page will be supported if Intel IOMMU
has the capability. With this option, super page will
not be supported.
ecs_off [Default Off]
By default, extended context tables will be supported if
the hardware advertises that it has support both for the
extended tables themselves, and also PASID support. With
this option set, extended tables will not be used even
on hardware which claims to support them.
tboot_noforce [Default Off]
Do not force the Intel IOMMU enabled under tboot.
By default, tboot will force Intel IOMMU on, which
could harm performance of some high-throughput
devices like 40GBit network cards, even if identity
mapping is enabled.
Note that using this option lowers the security
provided by tboot because it makes the system
vulnerable to DMA attacks.
對應的內核代碼
static int __init intel_iommu_setup(char *str)//caq:cmdline參數設定
{
if (!str)
return -EINVAL;
while (*str) {
if (!strncmp(str, "on", 2)) {//caq:開啟 intel_iommu 驅動
dmar_disabled = 0;
pr_info("IOMMU enabled\n");
} else if (!strncmp(str, "off", 3)) {//caq:關閉 inetl_iommu 驅動
dmar_disabled = 1;
pr_info("IOMMU disabled\n");
} else if (!strncmp(str, "igfx_off", 8)) {//caq:開啟則意味着：allows the GPU to access the physical memory directly without going through the DMAR
dmar_map_gfx = 0;
pr_info("Disable GFX device mapping\n");
} else if (!strncmp(str, "forcedac", 8)) {//caq:強制PCI設備使用DAC,從而禁止進行地址轉換
pr_info("Forcing DAC for PCI devices\n");
dmar_forcedac = 1;
} else if (!strncmp(str, "strict", 6)) {//caq:帶strict 參數，禁止批量刷寫IOTLB
pr_info("Disable batched IOTLB flush\n");
intel_iommu_strict = 1;
} else if (!strncmp(str, "sp_off", 6)) {//caq:關閉super page支持(默認值為開啟)，非4k的page叫超級頁
pr_info("Disable supported super page\n");
intel_iommu_superpage = 0;
} else if (!strncmp(str, "ecs_off", 7)) {//caq:ecs 就是 extended context
printk(KERN_INFO
"Intel-IOMMU: disable extended context table support\n");
intel_iommu_ecs = 0;
} else if (!strncmp(str, "pasid28", 7)) {//caq: pasid 支持
printk(KERN_INFO
"Intel-IOMMU: enable pre-production PASID support\n");
intel_iommu_pasid28 = 1;
iommu_identity_mapping |= IDENTMAP_GFX;
} else if (!strncmp(str, "tboot_noforce", 13)) {//caq:安全boot
printk(KERN_INFO
"Intel-IOMMU: not forcing on after tboot. This could expose security risk for tboot\n");
intel_iommu_tboot_noforce = 1;
}
str += strcspn(str, ",");
while (*str == ',')
str++;
}
return 0;
}