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[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] Re: [PATCH v10 12/17] vpci/header: emulate PCI_COMMAND register for guests
Hi Roger, Stewart, Roger Pau Monné <roger.pau@xxxxxxxxxx> writes: > On Thu, Oct 12, 2023 at 10:09:18PM +0000, Volodymyr Babchuk wrote: >> From: Oleksandr Andrushchenko <oleksandr_andrushchenko@xxxxxxxx> >> >> Xen and/or Dom0 may have put values in PCI_COMMAND which they expect >> to remain unaltered. PCI_COMMAND_SERR bit is a good example: while the >> guest's view of this will want to be zero initially, the host having set >> it to 1 may not easily be overwritten with 0, or else we'd effectively >> imply giving the guest control of the bit. Thus, PCI_COMMAND register needs >> proper emulation in order to honor host's settings. >> >> According to "PCI LOCAL BUS SPECIFICATION, REV. 3.0", section "6.2.2 >> Device Control" the reset state of the command register is typically 0, >> so when assigning a PCI device use 0 as the initial state for the guest's >> view >> of the command register. >> >> Here is the full list of command register bits with notes about >> emulation, along with QEMU behavior in the same situation: >> >> PCI_COMMAND_IO - QEMU does not allow a guest to change value of this bit >> in real device. Instead it is always set to 1. A guest can write to this >> register, but writes are ignored. >> >> PCI_COMMAND_MEMORY - QEMU behaves exactly as with PCI_COMMAND_IO. In >> Xen case, we handle writes to this bit by mapping/unmapping BAR >> regions. For devices assigned to DomUs, memory decoding will be >> disabled and the initialization. >> >> PCI_COMMAND_MASTER - Allow guest to control it. QEMU passes through >> writes to this bit. >> >> PCI_COMMAND_SPECIAL - Guest can generate special cycles only if it has >> access to host bridge that supports software generation of special >> cycles. In our case guest has no access to host bridges at all. Value >> after reset is 0. QEMU passes through writes of this bit, we will do >> the same. >> >> PCI_COMMAND_INVALIDATE - Allows "Memory Write and Invalidate" commands >> to be generated. It requires additional configuration via Cacheline >> Size register. We are not emulating this register right now and we >> can't expect guest to properly configure it. QEMU "emulates" access to >> Cachline Size register by ignoring all writes to it. QEMU passes through >> writes of PCI_COMMAND_INVALIDATE bit, we will do the same. >> >> PCI_COMMAND_VGA_PALETTE - Enable VGA palette snooping. QEMU passes >> through writes of this bit, we will do the same. >> >> PCI_COMMAND_PARITY - Controls how device response to parity >> errors. QEMU ignores writes to this bit, we will do the same. >> >> PCI_COMMAND_WAIT - Reserved. Should be 0, but QEMU passes >> through writes of this bit, so we will do the same. >> >> PCI_COMMAND_SERR - Controls if device can assert SERR. QEMU ignores >> writes to this bit, we will do the same. >> >> PCI_COMMAND_FAST_BACK - Optional bit that allows fast back-to-back >> transactions. It is configured by firmware, so we don't want guest to >> control it. QEMU ignores writes to this bit, we will do the same. >> >> PCI_COMMAND_INTX_DISABLE - Disables INTx signals. If MSI(X) is >> enabled, device is prohibited from asserting INTx as per >> specification. Value after reset is 0. In QEMU case, it checks of INTx >> was mapped for a device. If it is not, then guest can't control >> PCI_COMMAND_INTX_DISABLE bit. In our case, we prohibit a guest to >> change value of this bit if MSI(X) is enabled. > > FWIW, bits 11-15 are RsvdP, so we will need to add support for them > after the series from Stewart that adds support for register masks is > accepted. Stewart's series implement much better register handling than this patch. What about dropping this change at all in favor of Stewart's implementation? I'll be 100% okay with this. [...] -- WBR, Volodymyr
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