QEMU源码全解析 —— PCI设备模拟（11）

本文链接： https://blog.csdn.net/phmatthaus/article/details/135593367

本文深入解析QEMU中PCI设备模拟的实现，重点讲解pci_default_write_config函数如何设置BAR基址，并通过pci_update_mappings完成内存映射。通过这个过程，QEMU建立内存分派表，使得虚拟机能够正确处理MMIO访问。

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接前一篇文章：

通过前文，SeaBIOS就把所有设备的BAR基址赋值好了。接下来解析与BAR地址设置相关的代码。pci_default_write_config函数在hw/ pci /pci.c中，代码如下：


void pci_default_write_config(PCIDevice *d, uint32_t addr, uint32_t val_in, int l)
{
    int i, was_irq_disabled = pci_irq_disabled(d);
    uint32_t val = val_in;
 
    assert(addr + l <= pci_config_size(d));
 
    for (i = 0; i < l; val >>= 8, ++i) {
        uint8_t wmask = d->wmask[addr + i];
        uint8_t w1cmask = d->w1cmask[addr + i];
        assert(!(wmask & w1cmask));
        d->config[addr + i] = (d->config[addr + i] & ~wmask) | (val & wmask);
        d->config[addr + i] &= ~(val & w1cmask); /* W1C: Write 1 to Clear */
    }
    if (ranges_overlap(addr, l, PCI_BASE_ADDRESS_0, 24) ||
        ranges_overlap(addr, l, PCI_ROM_ADDRESS, 4) ||
        ranges_overlap(addr, l, PCI_ROM_ADDRESS1, 4) ||
        range_covers_byte(addr, l, PCI_COMMAND))
        pci_update_mappings(d);
 
    if (ranges_overlap(addr, l, PCI_COMMAND, 2)) {
        pci_update_irq_disabled(d, was_irq_disabled);
        memory_region_set_enabled(&d->bus_master_enable_region,
                                  (pci_get_word(d->config + PCI_COMMAND)
                                   & PCI_COMMAND_MASTER) && d->has_power);
    }
 
    msi_write_config(d, addr, val_in, l);
    msix_write_config(d, addr, val_in, l);
    pcie_sriov_config_write(d, addr, val_in, l);
}

pci_default_write_config函数首先将传过来的值写入PCI设备的配置空间，因为地址是BAR的地址，所以会调用pci_update_mapping s函数。该函数也在hw/pci/pci.c中，代码如下：


static void pci_update_mappings(PCIDevice *d)
{
    PCIIORegion *r;
    int i;
    pcibus_t new_addr;
 
    for(i = 0; i < PCI_NUM_REGIONS; i++) {
        r = &d->io_regions[i];
 
        /* this region isn't registered */
        if (!r->size)
            continue;
 
        new_addr = pci_bar_address(d, i, r->type, r->size);
        if (!d->has_power) {
            new_addr = PCI_BAR_UNMAPPED;
        }
 
        /* This bar isn't changed */
        if (new_addr == r->addr)
            continue;
 
        /* now do the real mapping */
        if (r->addr != PCI_BAR_UNMAPPED) {
            trace_pci_update_mappings_del(d->name, pci_dev_bus_num(d),
                                          PCI_SLOT(d->devfn),
                                          PCI_FUNC(d->devfn),
                                          i, r->addr, r->size);
            memory_region_del_subregion(r->address_space, r->memory);
        }
        r->addr = new_addr;
        if (r->addr != PCI_BAR_UNMAPPED) {
            trace_pci_update_mappings_add(d->name, pci_dev_bus_num(d),
                                          PCI_SLOT(d->devfn),
                                          PCI_FUNC(d->devfn),
                                          i, r->addr, r->size);
            memory_region_add_subregion_overlap(r->address_space,
                                                r->addr, r->memory, 1);
        }
    }
 
    pci_update_vga(d);
}

1）首先从pci_bar_address函数中得到BAR的地址。代码片段如下：


        new_addr = pci_bar_address(d, i, r->type, r->size);
        if (!d->has_power) {
            new_addr = PCI_BAR_UNMAPPED;
        }

2）然后复制到 io_regions中的addr。代码片段如下：


        /* now do the real mapping */
        if (r->addr != PCI_BAR_UNMAPPED) {
            trace_pci_update_mappings_del(d->name, pci_dev_bus_num(d),
                                          PCI_SLOT(d->devfn),
                                          PCI_FUNC(d->devfn),
                                          i, r->addr, r->size);
            memory_region_del_subregion(r->address_space, r->memory);
        }
        r->addr = new_addr;

3）最后调用memory_region_add_subregion_overlap函数将该地址添加到PCI MemoryRegion，作为它的一个子MR。代码片段如下：


        if (r->addr != PCI_BAR_UNMAPPED) {
            trace_pci_update_mappings_add(d->name, pci_dev_bus_num(d),
                                          PCI_SLOT(d->devfn),
                                          PCI_FUNC(d->devfn),
                                          i, r->addr, r->size);
            memory_region_add_subregion_overlap(r->address_space,
                                                r->addr, r->memory, 1);
        }

4）再经过内存提交，就会在QEMU中建立起内存分派表。从而在虚拟机访问这块MMIO内存的时候，由其对应设备的回调函数来处理。