| Message ID | 20200703173948.22156-1-alex.bennee@linaro.org (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | [qemu-web] blog post: Anatomy of a Boot, a QEMU perspective | expand |
On 03/07/20 19:39, Alex Bennée wrote: > While working on some test cases I realised there was quite a lot of > assumed knowledge about how things boot up. I thought it would be > worth gathering this together in a user facing document where we could > pour in the details and background to the boot process. The consensus > seems to be this is better as a blog post. > > The post follows the socratic method and leaves the reader to ask > themselves some questions in an effort to elucidate them about any > problems they may be having before telling them that they probably > wanted to use virt-manager all along. > > Based on Message-Id: <20200701161153.30988-2-alex.bennee@linaro.org> > Signed-off-by: Alex Bennée <alex.bennee@linaro.org> > [rth: review of doc form] > Reviewed-by: Richard Henderson <richard.henderson@linaro.org> Looks good to me. Paolo > > --- > v3 > - convert from doc to blog post > - address pm's comments > - new intro/outro for blog style > --- > _posts/2020-07-03-anatomy-of-a-boot.md | 161 +++++++++++++++++++++++++ > 1 file changed, 161 insertions(+) > create mode 100644 _posts/2020-07-03-anatomy-of-a-boot.md > > diff --git a/_posts/2020-07-03-anatomy-of-a-boot.md b/_posts/2020-07-03-anatomy-of-a-boot.md > new file mode 100644 > index 0000000..67aa72c > --- /dev/null > +++ b/_posts/2020-07-03-anatomy-of-a-boot.md > @@ -0,0 +1,161 @@ > +--- > +layout: post > +title: "Anatomy of a Boot, a QEMU perspective" > +date: 2020-07-3 18:00:00:00 +0000 > +last_modified_at: 2020-07-3 18:00:00:00 +0000 > +author: Alex Bennée > +categories: [boot, softmmu, system, firmware] > +--- > + > +Have you ever wondered about the process a machine goes through to get > +to the point of a usable system? This post will give an overview of > +how machines boot and how this matters to QEMU. We will discuss > +firmware and BIOSes and the things they do before the OS kernel is > +loaded and your usable system is finally ready. > + > +## Firmware > + > +When a CPU is powered up it knows nothing about its environment. The > +internal state, including the program counter (PC), will be reset to a > +defined set of values and it will attempt to fetch the first > +instruction and execute it. It is the job of the firmware to bring a > +CPU up from the initial few instructions to running in a relatively > +sane execution environment. Firmware tends to be specific to the > +hardware in question and is stored on non-volatile memory (memory that > +survives a power off), usually a ROM or flash device on the computers > +main board. > + > +Some examples of what firmware does include: > + > +### Early Hardware Setup > + > +Modern hardware often requires configuring before it is usable. For > +example most modern systems won't have working RAM until the memory > +controller has been programmed with the correct timings for whatever > +memory is installed on the system. Processors may boot with a very > +restricted view of the memory map until RAM and other key peripherals > +have been configured to appear in its address space. Some hardware > +may not even appear until some sort of blob has been loaded into it so > +it can start responding to the CPU. > + > +Fortunately for QEMU we don't have to worry too much about this very > +low level configuration. The device model we present to the CPU at > +start-up will generally respond to IO access from the processor straight > +away. > + > +### BIOS or Firmware Services > + > +In the early days of the PC era the BIOS or Basic Input/Output System > +provided an abstraction interface to the operating system which > +allowed the OS to do basic IO operations without having to directly > +drive the hardware. Since then the scope of these firmware services > +has grown as systems become more and more complex. > + > +Modern firmware often follows the Unified Extensible Firmware > +Interface (UEFI) which provides services like secure boot, persistent > +variables and external time-keeping. > + > +There can often be multiple levels of firmware service functions. For > +example systems which support secure execution enclaves generally have > +a firmware component that executes in this secure mode which the > +operating system can call in a defined secure manner to undertake > +security sensitive tasks on its behalf. > + > +### Hardware Enumeration > + > +It is easy to assume that modern hardware is built to be discoverable > +and all the operating system needs to do is enumerate the various > +buses on the system to find out what hardware exists. While buses like > +PCI and USB do support discovery there is usually much more on a > +modern system than just these two things. > + > +This process of discovery can take some time as devices usually need > +to be probed and some time allowed for the buses to settle and the > +probe to complete. For purely virtual machines operating in on-demand > +cloud environments you may operate with stripped down kernels that > +only support a fixed expected environment so they can boot as fast as > +possible. > + > +In the embedded world it used to be acceptable to have a similar > +custom compiled kernel which knew where everything is meant to be. > +However this was a brittle approach and not very flexible. For example > +a general purpose distribution would have to ship a special kernel for > +each variant of hardware you wanted to run on. If you try and use a > +kernel compiled for one platform that nominally uses the same > +processor as another platform the result will generally not work. > + > +The more modern approach is to have a "generic" kernel that has a > +number of different drivers compiled in which are then enabled based > +on a hardware description provided by the firmware. This allows > +flexibility on both sides. The software distribution is less concerned > +about managing lots of different kernels for different pieces of > +hardware. The hardware manufacturer is also able to make small changes > +to the board over time to fix bugs or change minor components. > + > +The two main methods for this are the Advanced Configuration and Power > +Interface (ACPI) and Device Trees. ACPI originated from the PC world > +although it is becoming increasingly common for "enterprise" hardware > +like servers. Device Trees of various forms have existed for a while > +with perhaps the most common being Flattened Device Trees (FDT). > + > +## Boot Code > + > +The line between firmware and boot code is a very blurry one. However > +from a functionality point of view we have moved from ensuring the > +hardware is usable as a computing device to finding and loading a > +kernel which is then going to take over control of the system. Modern > +firmware often has the ability to boot a kernel directly and in some > +systems you might chain through several boot loaders before the final > +kernel takes control. > + > +The boot loader needs to do 3 things: > + > + - find a kernel and load it into RAM > + - ensure the CPU is in the correct mode for the kernel to boot > + - pass any information the kernel may need to boot and can't find itself > + > +Once it has done these things it can jump to the kernel and let it get > +on with things. > + > +## Kernel > + > +The Kernel now takes over and will be in charge of the system from now > +on. It will enumerate all the devices on the system (again) and load > +drivers that can control them. It will then locate some sort of > +file-system and eventually start running programs that actually do > +work. > + > +## Questions to ask yourself > + > +Having given this overview of booting here are some questions you > +should ask when diagnosing boot problems. > + > +### Hardware > + > + - is the platform fixed or dynamic? > + - is the platform enumeratable (e.g. PCI/USB)? > + > +### Firmware > + > + - is the firmware built for the platform you are booting? > + - does the firmware need storage for variables (boot index etc)? > + - does the firmware provide a service to kernels (e.g. ACPI/EFI)? > + > +### Kernel > + > + - is the kernel platform specific or generic? > + - how will the kernel enumerate the platform? > + - can the kernel interface talk to the firmware? > + > +## Final Thoughts > + > +When users visit the IRC channel to ask why a particular kernel won't > +boot our first response is almost always to check the kernel is > +actually matched to the hardware being instantiated. For ARM boards in > +particular just being built for the same processor is generally not > +enough and hopefully having made it through this post you see why. > +This complexity is also the reason why we generally suggest using a > +tool like [virt-manager](https://virt-manager.org/) to configure QEMU > +as it is designed to ensure the right components and firmware is > +selected to boot a given system. > + >
On 07/07/2020 12.33, Paolo Bonzini wrote: > On 03/07/20 19:39, Alex Bennée wrote: >> While working on some test cases I realised there was quite a lot of >> assumed knowledge about how things boot up. I thought it would be >> worth gathering this together in a user facing document where we could >> pour in the details and background to the boot process. The consensus >> seems to be this is better as a blog post. >> >> The post follows the socratic method and leaves the reader to ask >> themselves some questions in an effort to elucidate them about any >> problems they may be having before telling them that they probably >> wanted to use virt-manager all along. >> >> Based on Message-Id: <20200701161153.30988-2-alex.bennee@linaro.org> >> Signed-off-by: Alex Bennée <alex.bennee@linaro.org> >> [rth: review of doc form] >> Reviewed-by: Richard Henderson <richard.henderson@linaro.org> > > Looks good to me. Looks good to me, too. Pushed. Thanks, Thomas
diff --git a/_posts/2020-07-03-anatomy-of-a-boot.md b/_posts/2020-07-03-anatomy-of-a-boot.md new file mode 100644 index 0000000..67aa72c --- /dev/null +++ b/_posts/2020-07-03-anatomy-of-a-boot.md @@ -0,0 +1,161 @@ +--- +layout: post +title: "Anatomy of a Boot, a QEMU perspective" +date: 2020-07-3 18:00:00:00 +0000 +last_modified_at: 2020-07-3 18:00:00:00 +0000 +author: Alex Bennée +categories: [boot, softmmu, system, firmware] +--- + +Have you ever wondered about the process a machine goes through to get +to the point of a usable system? This post will give an overview of +how machines boot and how this matters to QEMU. We will discuss +firmware and BIOSes and the things they do before the OS kernel is +loaded and your usable system is finally ready. + +## Firmware + +When a CPU is powered up it knows nothing about its environment. The +internal state, including the program counter (PC), will be reset to a +defined set of values and it will attempt to fetch the first +instruction and execute it. It is the job of the firmware to bring a +CPU up from the initial few instructions to running in a relatively +sane execution environment. Firmware tends to be specific to the +hardware in question and is stored on non-volatile memory (memory that +survives a power off), usually a ROM or flash device on the computers +main board. + +Some examples of what firmware does include: + +### Early Hardware Setup + +Modern hardware often requires configuring before it is usable. For +example most modern systems won't have working RAM until the memory +controller has been programmed with the correct timings for whatever +memory is installed on the system. Processors may boot with a very +restricted view of the memory map until RAM and other key peripherals +have been configured to appear in its address space. Some hardware +may not even appear until some sort of blob has been loaded into it so +it can start responding to the CPU. + +Fortunately for QEMU we don't have to worry too much about this very +low level configuration. The device model we present to the CPU at +start-up will generally respond to IO access from the processor straight +away. + +### BIOS or Firmware Services + +In the early days of the PC era the BIOS or Basic Input/Output System +provided an abstraction interface to the operating system which +allowed the OS to do basic IO operations without having to directly +drive the hardware. Since then the scope of these firmware services +has grown as systems become more and more complex. + +Modern firmware often follows the Unified Extensible Firmware +Interface (UEFI) which provides services like secure boot, persistent +variables and external time-keeping. + +There can often be multiple levels of firmware service functions. For +example systems which support secure execution enclaves generally have +a firmware component that executes in this secure mode which the +operating system can call in a defined secure manner to undertake +security sensitive tasks on its behalf. + +### Hardware Enumeration + +It is easy to assume that modern hardware is built to be discoverable +and all the operating system needs to do is enumerate the various +buses on the system to find out what hardware exists. While buses like +PCI and USB do support discovery there is usually much more on a +modern system than just these two things. + +This process of discovery can take some time as devices usually need +to be probed and some time allowed for the buses to settle and the +probe to complete. For purely virtual machines operating in on-demand +cloud environments you may operate with stripped down kernels that +only support a fixed expected environment so they can boot as fast as +possible. + +In the embedded world it used to be acceptable to have a similar +custom compiled kernel which knew where everything is meant to be. +However this was a brittle approach and not very flexible. For example +a general purpose distribution would have to ship a special kernel for +each variant of hardware you wanted to run on. If you try and use a +kernel compiled for one platform that nominally uses the same +processor as another platform the result will generally not work. + +The more modern approach is to have a "generic" kernel that has a +number of different drivers compiled in which are then enabled based +on a hardware description provided by the firmware. This allows +flexibility on both sides. The software distribution is less concerned +about managing lots of different kernels for different pieces of +hardware. The hardware manufacturer is also able to make small changes +to the board over time to fix bugs or change minor components. + +The two main methods for this are the Advanced Configuration and Power +Interface (ACPI) and Device Trees. ACPI originated from the PC world +although it is becoming increasingly common for "enterprise" hardware +like servers. Device Trees of various forms have existed for a while +with perhaps the most common being Flattened Device Trees (FDT). + +## Boot Code + +The line between firmware and boot code is a very blurry one. However +from a functionality point of view we have moved from ensuring the +hardware is usable as a computing device to finding and loading a +kernel which is then going to take over control of the system. Modern +firmware often has the ability to boot a kernel directly and in some +systems you might chain through several boot loaders before the final +kernel takes control. + +The boot loader needs to do 3 things: + + - find a kernel and load it into RAM + - ensure the CPU is in the correct mode for the kernel to boot + - pass any information the kernel may need to boot and can't find itself + +Once it has done these things it can jump to the kernel and let it get +on with things. + +## Kernel + +The Kernel now takes over and will be in charge of the system from now +on. It will enumerate all the devices on the system (again) and load +drivers that can control them. It will then locate some sort of +file-system and eventually start running programs that actually do +work. + +## Questions to ask yourself + +Having given this overview of booting here are some questions you +should ask when diagnosing boot problems. + +### Hardware + + - is the platform fixed or dynamic? + - is the platform enumeratable (e.g. PCI/USB)? + +### Firmware + + - is the firmware built for the platform you are booting? + - does the firmware need storage for variables (boot index etc)? + - does the firmware provide a service to kernels (e.g. ACPI/EFI)? + +### Kernel + + - is the kernel platform specific or generic? + - how will the kernel enumerate the platform? + - can the kernel interface talk to the firmware? + +## Final Thoughts + +When users visit the IRC channel to ask why a particular kernel won't +boot our first response is almost always to check the kernel is +actually matched to the hardware being instantiated. For ARM boards in +particular just being built for the same processor is generally not +enough and hopefully having made it through this post you see why. +This complexity is also the reason why we generally suggest using a +tool like [virt-manager](https://virt-manager.org/) to configure QEMU +as it is designed to ensure the right components and firmware is +selected to boot a given system. +