I'm trying to refactor some arch-specific code into common code and was
surprised to find out that the x86 domain structure already occupies
PAGE_SIZE bytes, couldn't even add an unsigned short field in it w/o
causing a compile-time error.
I'm using the master branch of git://xenbits.xenproject.org/xen.git,
compiled Xen by simply running "./configure && make dist-xen" (on my
x86_64 Ubuntu machine).
Concretely, the change that caused problems was the refactoring of the
x86 arch_domain.monitor field [@ xen/include/asm-x86/domain.h], which
originally was:
struct {
unsigned int write_ctrlreg_enabled : 4;
unsigned int write_ctrlreg_sync : 4;
unsigned int write_ctrlreg_onchangeonly : 4;
unsigned int mov_to_msr_enabled : 1;
unsigned int mov_to_msr_extended : 1;
unsigned int singlestep_enabled : 1;
unsigned int software_breakpoint_enabled : 1;
unsigned int guest_request_enabled : 1;
unsigned int guest_request_sync : 1;
} monitor;
by leaving there only the x86-specific part, i.e.:
struct {
uint8_t mov_to_msr_enabled : 1;
uint8_t mov_to_msr_extended : 1;
} monitor;
and moving the rest directly into the domain structure, i.e. add @ the
end of struct domain [@ xen/include/xen/sched.h]:
struct {
uint16_t write_ctrlreg_enabled : 4;
uint16_t write_ctrlreg_sync : 4;
uint16_t write_ctrlreg_onchangeonly : 4;
uint16_t singlestep_enabled : 1;
uint16_t software_breakpoint_enabled : 1;
uint16_t guest_request_enabled : 1;
uint16_t guest_request_sync : 1;
} monitor;
After the above change, X86 compilation of [xen/arch/x86/domain.c] fails w/:
error: static assertion failed: "!(sizeof(*d) > PAGE_SIZE)"
the line that caused the fail being:
BUILD_BUG_ON(sizeof(*d) > PAGE_SIZE);
To investigate I compiled the unaltered domain.c & used the pahole tool
(part of dwarves package) to obtain the complete layout of the domain
structure (& its members).
What I obtained:
* sizeof(struct domain) is already = 4096 bytes (= PAGE_SIZE)
* sizeof(struct arch_domain) [x86] = sizeof(domain.arch) = 3328
bytes (arch_domain is marked __cacheline_aligned, i.e. aligned to 128 bytes)
* sizeof(domain.arch.hvm_domain) = 2224 bytes
* sizeof(domain.arch.hvm_domain.pl_time) = 1088 bytes
=> overall, X86 timers-related information occupies the most.
One could shrink the domain structure by transforming some of its fields
to pointers, e.g. I could transform the pl_time field into a pointer and
dynamically allocate its data when domain_create is called.
Since the domain structure was designed to fit in a single page &
arch_domain is marked __cacheline_aligned I presume such changes are
sensible and should be done wisely.
How I should proceed?
Thank you,
Corneliu.
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